You're right on both counts: aging spiced beers will definitely cause the spices to mellow over time, and a later addition - after the beer has conditioned for a while - will avoid this issue.
The problem with a late addition, though, is that adding spices exposes your beer to oxygen. Doing so after an extended period of conditioning means that there are fewer yeast cells left in solution to consume the oxygen and prevent staling.
In my opinion, the best solution is to add enough spices later on. That takes some practice and experimentation, but it's really the best answer.
As for your beer, I wouldn't worry to much. Vanilla isn't a delicate flavor like apricot, so it shouldn't vanish. Instead, it should mellow into the other flavors, which is desirable.
Primary 1: Hasty IPA
Secondary: Soured Golden
Kegged: American Wheat
Bottled: Belgian Golden Ale.
Planning: American Amber