Biden could come out looking like a winner, by simply NOT looking too foolish. This could be a trap for Ryan. The expectations for Biden could not be any lower.
Biden will probably try to play Reagan to Ryan as Mondale. But, I don't think that will work. The best Biden can do is try to play the friendly grandfather type, making Ryan look "mean" if he criticizes Biden.
I presume that Ryan is prepared for this. If Biden does indeed try to play the "age" card, I think Ryan's best approach would be to phrase his criticisms so they aren't perceived as directed at Biden. Point them at Obama, Pelosi, Reid, the Democrat-controlled Senate, etc. But, not Biden.
When (not if) Biden says something stupid, correct him gently. Romney did the right thing with Obama -- taking him down a couple of notches and puncturing his aura of invincibility was the right thing to do. But, there's no need to do that to Biden. Treat him like the crazy uncle at Thanksgiving dinner.
The moderator will try to hit Ryan at his perceived weak area being foreign policy. They will not go for domestic policy. They need a life line to Biden.