Biden may surprise; he might already know the questions whereas Ryan will have to come up with answers. I am still confident in Ryan, but don’t underestimate the opponent. Plugs can be slick. Just look at how long he’s been a politician.
Biden’s career goes back way before his 4 years as Obama’s VP. His stint with Obama has hurt his reputation. To avoid more damage to his legacy he might find himself unable to support some of Obama’s ideas in the debate, causing him to embarrass Obama to avoid further humiliating himself.