You are right. Remember in 2004 when John Edwards was going to bring all of his voters over? Well, he did not make a blip of difference because no one votes for VP.
I do however believe that McCain can make a difference by his choice of VP. But not by “bringing votes over”, but simply by picking a candidate that is an acknowledgment that he needs conservatives.
Conservatives won’t vote for McCain because they’ll get a conservative VP, they will reconsider McCain because McCain will have made a point to pick someone they could live with.
I don’t see either democratic candidate having such a hole in their base that could be filled by another VP. Since Hillary was somewhat for the war at one time, she would probably want to pick a reliable anti-war southerner. And Obama would likely want to pick someone with experience.
But what they won’t likely do is pick a pro-war moderate in order to balance their far-left anti-war liberalism. Even though that could possibly save them a state or two. I’m not sure there are any such candidates out there who would take the slot — although I haven’t ruled out Hillary picking James Webb, who at least is a military guy but not pro-war.