Oh, this is good. I thought a lot about this many years ago. Well, maybe not whether I like chocolate or vanilla, but what percentage of the population prefers one or the other. My thoughts then branched out into how many people prefer white cars over any other color. Don't ask, the questions are related trust me. Vanilla seems bland, right? White cars seem bland, also correct, right? Ok, I looked this up: 20% of all cars are white. Stay with me now, focus. I know, that leaves 80% other non-white type colors. However, no single color other than white achieves 20% ranking. Ergo, white is most popular.
Now, back to chocolate and vanilla. I researched this question and the current results are: vanilla gets 29% of the vote for most popular ice cream flavor. You can go to http://www.makeicecream.com/15mospopicec.html and they have the comprehensive list covering pretty much every flavor.
I think the reason for the popularity of white cars and vanilla ice cream is that there are so many colors and so many flavors that many people love that when the totals who love each one are added up, white and vanilla are logically bound to be the winners. It's hard to "hate" white paint or vanilla. You can like another color or flavor more, but no one hates these. Therefore, while people may have strong second choices, the non-threatening white paint and very pleasant vanilla will come out on top.
There ya go, I know you were dying to find out the reasons behind the choices.