I think it is clear that Joe Biden won the VP debate hands down. Unlike Sarah Palin most of the time, Biden actually answered the questions. He also gave effective rebuttals to Palin's attacks while her typical rebuttal was to say "No, I disagree," and then change the subject.
Honestly, you can't expect to "win" a debate if you don't even answer the questions being asked by the moderator. On several occasions Palin simply ignored the question posed by Gwen Ifill and launched into some prepared statement that was totally unrelated to the topic at hand. It got old quick.
I used to debate in high school. Then I judged debates at UIL speech competitions years later. You can't just ignore the topic at hand and talk about whatever you want to talk about. Palin may not have fallen flat, like many people feared (or hoped), but exceeding such low expectations does not automatically result in a win.
The early polling showed a clear victory for Biden. I expect that won't change, but I also don't expect it will change many minds one way or another either.