Adding Vanilla during secondary or bottling?

I made a pumpkin ale that is currently fermenting. I have a brewer friend who suggested adding some vanilla during bottling. After a bit of research, it seems that's not uncommon. Especially if I am wanting it to be fairly sweet.

Why would I not add it during secondary? Would the vanilla ferment rather than sweeten the beer?

I'm also a little skeptical due to explosive bottles. Would I want to use less priming sugar, to compensate?

put a couple of drop in each bottle... should be good to go... thats what I did