I know this probably isn't exactly suitable for you because of your weak PC but I had to throw this out some time.
I wasn't exactly satisfied with the vault house mods currently out there. RR Companions Vault is pretty cool, but was not exactly to my taste because I felt it was a bit out of place. It is still a very well designed mod though. Still, I wanted something big.
So I started my own project. The big problem was that I svck at interior designing. Luckily I stumbled upon http://www.fallout3nexus.com/downloads/file.php?id=9906. It's a huge vault, posted as a modder's resource. It took me quite a while before I didn't get lost in it anymore, even in the GECK! This vault is fully decorated and has all sorts of cool little things. It is unfinished, which is why it's a modder's resource, but it's one hell of a good modder's resource.
So I intend to put it to good use by turning it into a quest mod. To sum it up quickly, at some point the player can become the overseer of Vault 88, and it kind of becomes a Vault overseer simulator from there. The player overseer has to make decisions for the vault with regards to the following simplified variables: Happiness, Safety/Security (Guards, Healthcare), Working Resources (for getting jobs done) and Primary Supplies (food, water, clothes, etc.). The better things go, the more people will come to the vault. No more loneliness.
I only started this recently though, so I'm not very far. So far I've been preparing Vault 88 to be able to hold a decent number of NPCs without slowing the game down too much, especially for lower end PCs. Part of the Atrium level was separated from the rest and put in a separate level (which was, because there are windows everywhere and often two ways into pretty much any area, a bit difficult) to improve performance. And pathfinding had to be added everywhere.
But I've got a pretty good idea of the different quests I want to offer, how the different variables will influence each other, and most importantly how to script it. It's going to be a challenge for sure, but I think I can do it.
I do recommend people checking out the http://www.fallout3nexus.com/downloads/file.php?id=9906, it's quite fun to explore even if there's nothing else to do.