As Harry points out, distillation proof is everything here. Most vodka comes off the still at something close to 100% pure grain alcohol (or as close as the distiller can make it practiceable). For example, I have sampled vodka at the top of Buffalo Trace's vodka still at 190 proof (95% abv). The reason for this is to eliminate flavor elements from the liquid. In the U.S., at least, vodka must legally have no 'distinctive' flavor or aroma. In other words, you shouldn't be able to tell vodka made from corn from vodka made from rye, wheat or potatoes (and there are plenty of blind-tasting results which indicate you really can't, but that's a debate for another thread/forum).
Whisky deliberately is 'proofed out' at a lower ABV -- not more than 160 proof (80% abv) in the U.S., for example -- in order to retain the congeneric flavors of the various grains, yeast, etc. So, while new-barrel aging can impart well over half of the flavor of bourbon, it's still not that rare to find non-expert tasters who can almost uniformly tell a wheated bourbon from a tradition rye bourbon or straight rye based simply on the flavors inherent to the grains.