How to neutralize the flavor of 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.