I'd be happy to see Biden entry the fray. He's linked with a fairly successful Obama administration, and I think he deserves credit for having been the administration's conscience on gay marriage. As for the impact on Hillary Clinton, I'm not sure. Possibly he'd take some wind out of her sails, since he'd potentially split the votes of the Obama coalition, which seem to be falling mostly to her right now. But it's also possible that he'd just split the attention that could otherwise be capitalized on by some other less popular candidate, like Sanders, making it even easier for Clinton to coast on her absurdly large lead in the polls all the way to the nomination.
As for the idea that "she does not seem up to the rigors of the campaign" and that "she's already not doing very well," what you need to do is step outside of the right-wing-media news bubble every once in a while to breath the fresh air of reality. Love her or hate her, Hillary Clinton is dominating the Democratic nomination fight as few non-incumbents have ever dominated. Right now, in the RCP polling average, she leads by 24.3 points! The closest I've seen anyone in any major poll is Sanders 18 points behind her in a CNN poll.
To put it in perspective, Trump, who is considered to be doing very well right now in the Republican field, is polling at an average of 22.0. Clinton is polling at an average of 49.3. She's doing well over twice as well as a guy who is seen as dominant on the Republican side.
I'm always fascinated by this right-wing alternative reality. I'm not sure when the last time a non-incumbent primary candidate had this large a share of support at this stage in a primary fight. For example, at this point in 2011, Romney was polling at 17%. At this point in 2007, the Republican field was led by Giuliani with 28%. Has any non-incumbent candidate EVER been doing as well, at this stage, as Hillary Clinton is currently doing?! Yet, despite that historic level of dominance, those who have entombed themselves in the right-wing media bubble have been told over and over that she's not doing well, and they buy it.