Ah. There are two actual definitions of "Vanilla" game.
betteroblivion.com says:
... the term "Vanilla Oblivion" is used frequently, or simply "vanilla". Vanilla Oblivion is Oblivion without the addition of mods.
That is what I take as the official definition, the game without mods.
However, a lot of people seem to think it means the pure game, without mods
or DLC Add-ons. Complete virgin game. This is not the true definition of vanilla. Oblivion with SI, KotN and other add-ons still counts as Vanilla Oblivion.
I just wanted to clear that little thing up for everyone on here, since it's been causing confusion for a while.