I'm not sure that he's going to bomb. He might not--he might actually hold his own. In which case, the polls won't show much more than a blip downwards, a point or two; and the 'surprise' won't be too overt. If Biden absolutely tanks, then there'll be an alpha strike on the Iranian nuclear facilities on the weekend, when the moon is new.
Im pretty sure Biden is in deep horsepucky.
Biden's rumored "strength" is on foreign policy - If I recall, that's the reason Obama brought him in.
I suspect that the Benghazi SNAFU has derailed any possibility that the debate will focus on international matters for more than a few milliseconds. Leaving what ? Economics ? Ryan will hand Biden his ass on that. So then what ? All I can think of is designing the debate to find new reasons to hate old, rich, white men, as well as making certain to rehash all the old reasons to hate old, rich, white men. Unfortunately for Biden - he certainly falls within that category.
As DU would say, he is well and truly screwn.