I wouldn't reccommend boiling the beans, personally. During primary fermentation, the CO2 from the yeast is going to push a lot of the vanilla aroma out of the beer. Wait until primary is done and then add the vodka soaked beans for another week or strain the vodka from the beans and add the liquid at bottling/kegging.
If you're using beans, cut them lengthwise, and scrape out the inner goodness and add enough vodka to cover it all. Anymore than 8oz. of hard liquor in 5 gallons of beer can be quite overwhelming, so less is more. I would use 2-3 beans for what you're attempting.