People who clamor for vanilla servers generally want two things:
- Vanilla content - the quests, zones, and classes that existed at that time.
- Vanilla "feel" - a return to an era where server community was important, where guild and reputation mattered, where the difficulty was slightly higher and required groups to interact more.
Of these two, I think getting the first one is extraordinarily unlikely. Blizzard would essentially be launching and maintaining a completely separate game. I think that the risk is too high. So given that this is basically not going to happen, I don't see much point in expending energy over it.
The second item, however, I believe might be possible. It would be a variant on the current game, and would be a lot easier to maintain. Fixes for the current game would hit both server types. It might even be good for the game by segregating the audience a little bit. The people who insist on increased challenge would have a home.
Of course, even maintaining server variants is more effort. There's already two variants in PvP/PvE. Another orthogonal variant makes four possibilities. But I think it's still less risky and less effort than a full vanilla server.