I have good experiences with fully roleplay servers. Do you know Ultima Online? Lots of shards works as roleplay ones.
I would play on fully roleplay fallout server.
But there are some problems.
The first one is rules and "cops", someone have to protect community against trolls. Maybe start it as closed server, and slowly recruit new players, friends of those players. So community would be pretty closed, happy etc. and then, when everyone would know everyone, the server could go open, because community would be settled and trying itself to protect its rules against trolls.
Another problem is too mutch of space. Because roleplaying is about player interaction, you cannot build roleplay server on big world, or you can, but there should be only few places, where you can live.
As the last thing, I would likely to try some oneshot. Like that vault. Two hours project with players as actors, or something like P'n'P RPG in fonline enviroment. Or urban larp...