I was inspired by this mod https://www.nexusmods.com/fallout3/mods/10834 which originally I was going to use as a resource to copy but could not figure out how to do it, without needing people to download both that mod and my mod. So I created a series of Pulowski vaults and placed one under every existing Pulowski shelter apart from 2; The one in the car park at Rivet City's stern because that has a great mod for it, and the one at Paradise Falls. 

So, the backstory:

You've all heard of Vault-tek, maker of high quality and high price vaults. Well it has a lesser know rival Pulowski. Oh yes, you've probably seen their porta shelters at Metro station entrances and around the place, but I bet you didn't know they also made vaults. Unlike their more expensive competitors Pulowski uses mass production, and basic designs to produce Pulowski Shelters at half the cost of Vault-Tek. Pulowski Sheters use modular design to reduce costs. Yes they're cheap and cheerful, but they'll protect you just as well as a Vaul-Tek vault!

Pulowski Shelters were installed at Metro Stations and public places. Cheaper than Vault-Tek vaults, communities and town councils could afford to buy small community vaults. The city paid for larger community vaults to be installed. Stocked with basic food supplies, water, seating, bedding, first aid kits and cooking facilities, the Pulowski Shelters had everything needed to keep the occupants safe from fallout and damage from bombs. Hundreds of people owe their lives to these shelters. 94% of shelters survived the Great War intact.

After the war many of the shelters were used as temporary housing, with some still occupied. Most were restocked and locked up by the military or their owners after the war due to fears of ongoing conflict.

Locations:
Almost every Metro station, in most towns and various places.
Most are locked but you can find keys all over the place including in Pulowski shelters themselves, close by, and in Moriarity's and Crater Supply.

Bug report.
I never navmeshed these bunkers, so your companions may get temporarily trapped in them. Just fast travel to another location and your companions should appear. 


Lore 
I tried to make them lore friendly. Okay, I realise most of the supplies would have been ransacked in reality, hence why they are locked up and the backstory about them being restocked after the war.

Update: 24-11-2020.
I made a minor change to fix an issue with "The Gun" not firing fast enough. I replaced it with "That Gun" which fixes the issue. That Gun is a kick ass sniper rifle that has a bigger magazine and does 400 Hp of damage! :) Basically it fires explosive rounds.

Credits
Credit to MTB for making https://www.nexusmods.com/fallout3/mods/10834 which inspired me.
And to Bethsoft for the GECK and FO3.