Give Cheney his props. Use his momentum against him: He's obviously smarter and more capable than his boss.
Whenever a Democratic surrogate is confronted with a question like: "A lot of people are saying Dick Cheney performed quite well last night and some are saying he won the debate outright, what did you think of the VP's performance?" The response should be:
"I too was impressed by Mr. Cheney's performance last night. After the President's, frankly pathetic performance in the first Presidential debate I was struck by how much more prepared and knowledgeable Mr. Cheney was compared to his boss. If you ask me, the GOP made a mistake and put the wrong guy at the top of the ticket."
Or
"If this election was all about who's going to be the next President of the Senate, then Dick Cheney did very well last night. But this election is about who will be the next leader of the free world. As long as the Republicans have George W. Bush at the top of their ticket, it's hard to take them seriously."