The best answer for you is that there is no right answer. Its personal preference.
One of my favorite beers is an ICE cold Breckenridge Vanilla Porter. God I still remember the first night I had that beer. I got rocked off of it, couldn't get enough.
However, I did try to clone it once. I used a standard porter recipe and then put in the secondary 1.5 halved vanilla beans soaked in vodka for 24 hours, then racked on top of them. The vanilla flavor was not all that much, if doing it again I'd do 3 beans. But that would be to suit my taste