It sounds so .... DUMB.
"Vanilla-wow".
"Vanilla Diablo".
"Vanilla Halo".
Vanilla Vanilla Vanilla
CAPITALIZED V at that as if it's an actual title of the game.
"Starcraft 2: Vanilla". Yeah...
Just call them original WoW or pre-BC or original D3 or whatever vanilla does NOT sound like you're referring the first version of the game it sounds like you're either referring to a My Little Pony version of any game or a bland, simple and unoriginal version of a game, like the simplest and most uninteresting version of a game.
Sounds dumb dumb DUMB.