Just a thought that I've been kicking around in my spare time. Maybe instil a turn limit (say each player adds 10 turns to the game length) and everyone has to build their vaults from one deck. Best vault wins. accidents and raiders could be cards you play against other players. Something like a cross between Munchkin and Betrayal at House on the Hill. Maybe with a pinch of monopoly.

Have you checked out any similar games?

In Puerto Rico, you build up your colony, purchasing facilities, gathering people to run the facilities, producing resources... It's an incredible game because it is hard to create a single winning strategy for it, so we don't tend to get a single player dominating it all the time. I recommend you play this before designing your vault game.

You could go "Carcassonne" and have the players construct a single vault between them, but while competing with one another for points.

Other games with a settler angle to them include Revolution (which has bidding via resources for actions, and people competing to populate the board with their settlers) and Lords of Waterdeep (types of settler, competing actions, building facilities, resources, quests)

I want to see a Facebook game.