Out of curiosity, is there any flavour that neutralizes or balances out with vanilla?
I think it depends on how much is too much vanilla. If your recipe called for a teaspoon of vanilla and you added a tablespoon, it probably won't be too big a deal. I often use more vanilla than is called for in recipes.
If you are looking for something to "balance" the flavor a little bit, you could maybe try salt or lemon juice/extract.
If you just prefer that vanilla not be the dominant flavor, there are plenty of things that go nicely with vanilla that would result in a more complex flavor. Personally, I love combining vanilla with the flavors of fruit or almonds.
OK, so I bought some Vanilla Porter beer that was WAY too strong. I took a couple of tbs of the beer added about 1/3 rd tsp of unsweetened baking chocolate, microwaved it to get it hot so I could mix them, and it did the trick. It cut the vanila way back and didn't really add much choc flavor at all.