Put them in secondary for sure - boiling will kill off most of the aromatics. When you brew the beer, split open the vanilla beans, scrape the inside, and put it in a jar with some decent bourbon or vodka. Add the whole thing to secondary, or as primary winds down. Depending on the quality of the beans, 2-3 pods makes a fairly strong impact on 5 gallons. Heck, you could even strain the bourbon and add it to the keg... that way you can dial in exactly how much vanilla you want. It does fade fairly quickly, so maybe start out a bit heavy.