Just out of curiosity, is there a reason you wouldn't do the vanilla in primary in a carboy and transfer to a barrel to get the flavors you're looking for there? Or are you just worried about getting any vanilla flavor in the barrel in general?
If I had a time machine I could probably be of more help here since I literally just finished bottling a vanilla bourbon porter that's been sitting in the barrel for the past week. I initially had it in primary for three weeks with two beans, and opted to add a third when transferring into secondary in the barrel. I'm pretty happy with the initial tasting notes, but there's still a ways to go.