Don't have a straight answer for you but I figured I'd offer my experiences.
I made a vanilla porter last year and added 2 cut and scraped vanilla beans to 1.5 oz of 12 yr Jameson for a few days. I added the entire mix to the beer when I racked to secondary. For months it tasted like a whiskey barrel aged porter with the faintest hint of vanilla that you could barely pick up on. I couldn't believe how much the whiskey flavor stood out considering it was just 1.5 oz to 5 gallons of beer. Needless to say I was very disappointed with the result especially because it was a birthday present for my girlfriend - she hates whiskey.
I'd be interested in hearing others experiences as I plan on brewing this again this year for her birthday. This time I just won't mention it in case it doesn't come out good again.