Minidancer:
Vanilla's tend to stick to arse or bum.
Not in the U.S. - you rarely hear either of those words. 'Ass' maybe or 'tail', but I've heard plenty of vanillas say 'bottom'.
'Bottom' is the word I prefer, but I try to use different terms in stories to break up the monotony, just as I try to use various words for a spank or for the sound it produces.