Simple little ESL flagged ESP (ideal for merging since there is only one record) to disable sun damage in worldspaces that shouldn't really be considered bright, but are for some reason. In Update.esm there's a formlist of worldspaces that are exempted from sun damage. It's used as a condition for the sun damage abilities in vanilla, Sacrosanct and most other vampire mods. It includes the likes of Sovngarde, Karthspire Redoubt, and a lot of other exterior worldspaces that aren't even on plane of Oblivion or anything but are supposed to be right here on Nirn. Those are included because the engine's not super intelligent about deciding what is and isn't "interior."

So Karthspire Redoubt's worldspace includes a lot of interior sections that are still in the same cell as the open-air sections. I guess they decided it's better to disable the exterior sun damage than to let vampires take damage in the interior sections. So those worldspaces are exempted but for some reason, the planes of Oblivion aren't exempted. So you can take sun damage in those areas and there aren't any interior cells to take refuge in. I think the Soul Cairn and Apocrypha should just be exempted entirely since they otherwise look like ideal places for a vampire to take refuge from the sun. As for Darkfall Passage, it is entirely interior so it doesn't make sense to leave it off the list. Of course generally speaking the light level is gonna be pretty low, but if you use mods that add light sources to the glowing mushrooms and stuff, I'm pretty sure you could take damage in there. Provided you use a vampire mod that enables shade-based regeneration I mean.

Doesn't require any particular mod but it may not be necessary for some vampire mods if they already dealt with this problem. I haven't tried all of them. If it doesn't work for your vampire mod it might be because that mod doesn't use the formlist the same way vanilla does. If you need help with a specific vampire mod let me know, I'll be interested to look into it.

And don't ask about changing light levels themselves or changing the sneaking parameters in a worldspace. That's much more complicated and probably requires a netscript plugin to hook the engine's memory, since there aren't many user-facing functions or variables involved in calculating light levels. You can get the lightlevel, but you can't set it as far as I can tell. So the only way you can influence it is by changing the weathers and light sources themselves. Not any way to change just the virtual "lightlevel" without touching all the visual variables that influence it. There are some global values used by sneaking parameters themselves though, see the "thief skills rebalance" optional plugin from Ordinator.