Hey Denny,
If you've already done the experiment, then I take it back. I would have expected the lower ABV of the beer to have less efficient extraction of flavor compounds than the high ABV vodka. Furthermore, it's far easier to periodically agitate the vodka/bean mixture than it is the entire fermenter without ill effects. I would have imagined that the beans would be quickly covered by yeast in suspension or sink to the bottom and end up buried in the yeast cake reducing their contact with the beer. Mosher and Strong also advocate the vodka extraction method, but given that your name deserves mention along with theirs this carries quite a bit less weight than it might for someone else.
Did you try full scale or just drops in a sample? I could definitely detect the vodka in the sample (and certainly in the jar full of vodka and beans), but not in the carbed beer after adding the mixture to a week or so before bottling and kegging. Did you add the vodka mix to directly to the keg? If so is it possible that in the fermenter any hot alcohols can gas off, but in the keg they remain trapped? The vodka isn't even enough to move the dial on ABV. I used probably about 60 ml (about two shots) of vodka for 22 liters of beer which is only adding 24 ml of alcohol to a container that already has 1.32 liters of alcohol in it, essentially a rounding error.
-Anthony