(derived from ice cream's standard basic flavour): humorous term for the enjoyment of "straightforward" sexual intercourse, most obviously in the routine missionary position. If the phrase is used, the implication is normally that the enjoyment is somewhat limited, when perhaps other "flavours" might have been welcomed .... (Note: See also "chocolate and vanilla")
A: "So, how's the new girlfriend?"
B: "Oh, all right."
A: "What's the problem?"
B: "She's only interested in plain vanilla."
A: "Hell, man, give her time. You didn't expect a blowjob on the first date, did you?!"