Traditionally and logically, the role of the candidate for Vice President in the vice presidential debate is to attack the other party's Presidential candidate.
Whereas the Presidential candidate tries to look presidential and likable, the vice presidential candidate is free to attack.
Paul Ryan can present a strong image of a better debater and a better choice for Vice President. But Joe Biden can deliver attacks on Mitt Romney very effectively.
However, Joe Biden like Barack Obama has a severe weakness: Obama's entire campaign is built on LYING about Mitt Romney and other Republicans in general. This makes it like shooting fish in a barrel to not only debunk what Biden is saying but humiliate Biden the way Romney humiliated Obama. Obama and Biden are way out on a limb making false arguments. So Paul Ryan can slice and dice Joe Biden in the debates if he is ready to be aggressive and indignant at the lying (which he is).
If Paul Ryan brings a gun to a knife fight (as one Obama strategist once put it), he will win the strategic goal of the debate. Joe Biden will deliver the Obama campaign's attack lines with great force, energy, charisma, and persuasion. Paul Ryan should be able to anticipate almost exactly what Joe Biden is gong to say.