What is does:
Short version: This mod makes vendors use a currency that makes sense to them.

Long version: This mod will make it so that vendors in faction specific locations(such as Camp McCarran or the 188 Trading Post) use currency from their faction. Now the NCR arms merchant at the 188 Trading Post will now take NCR dollars instead of bottle caps, and Dale Barton, a merchant that supplies the legion will now use Legion coins as his primary currency. Same applies for repairing equipment. If you go to Samuel Kerr at the 188 Trading post, you'll pay him in NCR dollars instead of caps.

With the new Faction Addon, the value of the NCR and Legion currency will now change based on what the player does. IE, if the player takes Nelson from the legion, the value of the legion currency goes down, while the value of the NCR currency goes up. I am almost certain that I didn't get everything that would cause the currency to fluctuate, and I'm not huge into economics (ironically so) so some of the effects may not be perfect. If you have any suggestions or comments on something you don't think is right, just let me know, and I'll look into it.

You are able to access the systems from this mod with some simple setup scripts in any mod you like. You don't even need to have it dependent on this one. And moreso, in this update, a system is in place to allow you to create your own currency, usable however you like. Creating a new world space, and don't want to use CAPS in it? Don't want to prevent caps from being used entirely though? Just use a few simple scripts, and you can now use whatever currency you like. Copies of these scripts are included in the download file that explain how to use them. If there is any confusion, just let me know and I'll try explaining things better. After all, teaching isn't my forte, I'm a programmer after all.

Addons:
Brotherhood of Steel Addon: Makes the Brotherhood use Brotherhood Scrip instead of caps for merchants and doctors.

Brotherhood of Steel TTW Addon: Makes the Brotherhood in the Capital Wasteland use the Brotherhood Scrip as well. Again, merchants use it and you get rewarded in it.

Dead Money Addon: Makes the Hologram Vendors in Dead Money take prewar money instead of using caps.

Faction Addon: Adds all the faction merchants to the system. Separated from the main file upon request of several users.

Metro Addon: Makes wasteland merchants use 5.56mm ammo as currency. Inspired by Metro series. Idea by vanionchist.

Metro TTW Addon: Makes the wasteland merchants and doctors in the Capital Wasteland use 5.556mm Ammo as currency. Idea again by vanionchist.

Mothership Zeta TTW Addon: Makes the crystals you find around the ship a useable currency. Somah will now take them instead of caps for repairing your gear. I've also added a few, 4 or 5, alien vending machines around the ship that use the currency as well. Think Commissaries in Lonesome Road, except no made up reason as to why they use bottle caps. They are all linked to one vendor chest, so when you buy from one, they are all affected.

New Vegas Addon: Add New Vegas Dollars as currency to be used in New Vegas alone. To enter the city you have to now have 400 New Vegas Dollars. The Securitron Greeters can all exchange your currency. The bartenders now take this currency. The receptionist at the NCR Embassy is now also able to exchange your currencies. As their is yet no way to tell the system what to specifically convert, or rather leave anything off of the list of installed currencies, you are able to convert from ALL of the currencies. IE, if you have the Mothership Zeta Addon installed, you could convert all your $NCR to Crystals if you really wanted to. Or if you feel like you want to exchange all your $NCR to Brotherhood Scrip, that is also an option. Also adds securitrons in the strip itself that'll convert your currencies from either caps, legion, or $NCR to New Vegas Dollars.

Mojave Addon: While the base mod adds the faction merchants to the system, the Mojave Addon adds the rest. Now almost all merchants in the Mojave will take a currency that makes sense for them. IE, people in Goodsprings will take anything as currency, but the Great Khans or Powder Gangers won't take $NCR, and the Followers won't take Legion Coins. Same applies for the repairmen.

Fallout: New California Addon: Makes the merchants in Union City take $NCR as payment. They all say no caps, only NCR Dollars but in the end the vanilla way just doesn't support that, and I know that Radian-Helix Media will never implement a way to if it involves making their incredible mod dependent on something. You end up paying in caps anyways. This addon changes this, though. Everything to do with money in Union City is now processed with this system. Don't think this means I dislike FNC entirely, I just dislike this one aspect of it, although I understand why they did it.

TSC Addon: Makes the TSC credits in the TSC New Vegas mod usable in this system. Suggestion by MysteryMachineX. I'm not the most familiar with this mod anymore, but I don't think I missed anything. If I have, just let me know.

Who does this mod effect?

This mod affects who ever you want it to based on the addon you download. The base file doesn't affect anyone anymore, but the faction addon adds the merchants associated with whatever main faction to the system, while the mojave addon adds almost everyone else. The New Vegas addon adds...well, new vegas to the system and so on so forth. 

Using this system in your own mod:

Using this system in your mod is fairly simple now. At the moment, only Merchants and Repairmen can use it, doctors are still pending a simple way, as the one I am using isn't something that can be done easily.

No need to use this mod as a dependency this way, or even a requirement. If the user doesn't have/want my mod, it will just default to the old system.

I am working on making a comprehensive list of references and a guide on what this system is capable of. That will be able to be downloaded from the files page as well when it's done. With all the different functions that I keep adding, it's making my current references file a bit of a mess, and it no longer makes sense to have a copy of that here.

Upcoming:
These are what I want to do with this mod in the future.

Make the currency value change based on in game events. NCR losing the war? There currency is worth less now because of it. Legion moving into Nevada and showing everyone how much safer they are? There currency value goes up. Caps will stay the same, for simplicity's sake (as if any thing in this is simple)

Create currencies for the Brotherhood of Steel to use internally, as well as Mr. House. I also never liked that the Brotherhood of Steel, who has historically made their own internal currency (granted in a game that not considered canon, but still). Same with Mr. House, how has the means to make casino chips in post apocalyptia, but still uses bottle caps at all the bars and vendors on the strip.

Make several vendors in the Mojave take different types of currency. Places like Goodsprings I think should be able to accept Caps and NCR money. Legion money will be rather rare to find some who will take it. Places in Freeside will take both NCR and Caps. The Van Graffs, depending on how far you get into that particular quest, will take caps and Legion. Crap like that.

Make doctors use a different currency. Doctors healing your rads/health/addiction are handled differently than bartering and repairing. I'm going to have to look at this a bit more to find the best way to do it.

Go through and find faction characters that give caps as a reward and change it to giving faction currency instead. This one is simple, but time consuming.

Make more currency conversion stations. I want to have several that will only do specific conversions. IE, one in Camp McCarran that will only convert Caps to $NCR and vice versa, having nothing to do with Legion coins. One in Cottonwood Cove that will convert only with Legion Coins to Caps and vice versa, with nothing to do with $NCR. Some others will take both.

Make some more faction specific vendors in certain areas. Right now, the legion has exactly one vendor(if you can really call Dale Barton that). I want to add some more. Quartermasters for the legion and the like. The NCR doesn't have that many, either for that matter.

If you find any vendors that obviously should be using one currency or another, shoot me a message letting me know. I'll look into it and see if it needs changing.

If you have any good suggestions, just shoot me a message. If it's a bad suggestion, just keep it to yourself.

FAQ
I haven't actually had any questions yet, so this is what I expect will be asked. XD

Q: Why?
A: Because I can.

Q: No, really, why would you do this to yourself?
A: It's always bothered me. I mean, the NCR is taking the time and money to print their own currency, and they don't even use it. And you're telling me that the Legion would take the time to source metal and press unique coinage to only accept bottle caps as payment?

Q: How does this work?
A: Fairy magic... Alright, if you want the long drawn out answer, I basically trick the system into using a different currency when you enter the barter menu. Basically, and as simply as I can put it, when select the option to enter the barter menu, a script checks how many caps you have, stores that number somewhere in the aether, removes all your caps, checks how much you have in the currency you are using be it Legion or NCR, adds that amount of caps back to you, goes through the items in the Merchants inventory and players inventory and changes the value of them accordingly (NCR money is worth %40 of Caps, and Legion Coins are worth %400 of caps). After you make your selection in items, it then checks how much money you have left, does some wizardy crap to determine how much of each currency you should have(if you have $235 left after bartering with the NCR, you should get back 2 $100, 1 $20, and 3 $5) and then adds back how many caps you had at the start of this whole long process. Finally, it resets the values of the items back to what they were before this started.

Q: What the hell was that?
A: See, that's what you get for making me answer you, imaginary person talking to me. All you need to know is when you enter barter mode, you'll be doing it with whatever currency makes sense.

Q: I don't think ______ should accept ______ as currency.
A: First off, not really a question, but whatever. Second, I chose this solely on location and how I think the NCR/Legion would do things. Everyone in the NCR complains about things like taxes, so if they take a cut of everything being sold in their jurisdiction, I doubt they would allow just anyone to use whatever they want as money. Same goes for legion, except instead of going to merchant jail, you'd probably end up lashed to a cross. That being said, if you think that someone is using the wrong currency, just let me know and tell me why you think that. If I find the argument convincing, I'll change it.

Q: Why didn't you just change Bottle Caps to whatever currency?
A: Because I wanted to use more than one currency type you knob! (This comment I actually have gotten before) Just changing the "Caps001" item in the GECK to something else will most certainly change the currency as a whole. That being said, it changes it for everyone. The way I did it, while long winded, I found to be the easiest and most effective. It also gave me migraines trying to figure out the conversion rate as that was never something that I found easy, so yeah... I think it was worth it, but it's up to you if you think it was or not.