Adding vodka (or rum or Whiskey or insert liquor here) will extract MORE flavor from the vanilla beans then if you didn't use them. This may before then desired.
Hint: start with half of the whole bean. Lest rest for a couple days in secondary. Carefully swirl before sampling. If it's at the level you want, great. If not add a little more and repeat. If you do over-do it, bottle or keg and let it rest a month or two. The vanilla should diminish with time, but it may take many months. I personally don't like to be hit over, he head with vanilla... rather like a subtle note in my beer. But that's the joy of home brewing. You get to make what you like.