"Undecideds" that felt John Kerry won the 2004 presidential debates were likely to shift their support towards him. However, the perception that Cheney beat Edwards in the VP debate was enough to keep "undecideds" from breaking towards Kerry, even if they thought Kerry clearly beat Bush. This combination of views–that Kerry beat Bush, but that Cheney beat Edwards–was the most commonly held perception among "undecideds" in 2004.
This result suggests that VP debates should be viewed as having the potential to either reinforce or attenuate the impact of the presidential debates on individual vote intention.
Meanwhile, Silver believes that Biden could help shrink the enthusiasm gap:
It seems plausible that roughly half of Mr. Romney’s postdebate bounce — which appears to in the range of three to four points on average in the polls — has resulted from the shift in enthusiasm. It will be Mr. Biden’s job in Danville to work on that half of the problem. Then Mr. Obama will need to improve his performance in the last two debates to get back the rest of what he lost in Denver.
(The latest iteration of the meme of the day via Dorsey Shaw)