I just got my score sheets from my entry of the vanilla porter into the National Homebrew Competition, Northeast region. It scored a 32 out of 50. That's officially in the "very good" range, but it wasn't enough to be in the top three of its category and proceed to the second round (there were 23 entries in the category, with score min/average/max of 26/35/43). The judges generally thought it was tasty but too heavy on the vanilla, and could possibly use a bit more malt flavor. Touches of astringency were noted (possibly from the vanilla), and one judge thought there was a bit of a solvent flavor.
Anyway, it's good feedback. Next year I'll use one vanilla bean instead of two, and maybe throw some more toasty/biscuity malts in the mash. A lot of people I've served it to have liked the vanilla flavor, but I think it could be toned down without losing that appeal.