An obselete Forge term for game-play in which the GNS mode is easily-accessible and requires few if any complex rules-techniques -- as opposed to Pervy. This was more generally expressed by the idea of Points of Contact, which concerns the degree to which System is explored.

Away from the Forge, 'vanilla' typically refers to a game played without house rules, or without incorporating significant changes outside of the game text. (Ex. "We were just playing vanilla D&D. No added spells or prestige classes or anything." or "Well, maybe that's true in your campaign, but in vanilla Vampire, it's not like that.")

References:

• Vanilla and Pervy
• Pervy in my head