It is an old GM saying, No plot survives first contact with the players. This is true for MUs as well to a degree, the thing to do is get a bunch of players on board with the theme you are trying to run and the aspects of that theme that you are focusing on.
I think the most important thing is to be a specific as possible, Post-Apocalyptic can mean a lot of things. there are a ton of post apoc novels where the basic theme is people trying to recreate normal life as best as they can so Vampire that wants to recreate the life he enjoyed is not going against it being post apoc but is a different flavor for a post apoc story where the focus is on immediate survival.
Dies the Fire and Walking Dead are both stories about communities trying to survive in a post apoc world but they have a completely different feel as well as a different cause of the apocalypse.
As for how to get them to buy in I would say that is two fold part of it is to run and emphasize things that are in line with your vision so that being active means buying in and the second is be willing to compromise in small ways, if someone really wants to make a home brew set up or even easier a still to produce hard liquor let them but keep it limited and make it a thing, alcohol will be a valued commodity, word will get around and other survivors will want this instead of seeing it as conflict OOCly use it as the spur to why the raiders in the next plot are bothering with this settlement.
Some drift will happen anyway since most RP on MUSHes is social, so I would put the focus on guiding the drift rather than eliminating it.