In the United States what is the limit to being vice pesident ?

What limit would that be?
IQ?
Age?

Yes, Biden could be VP again. There is no limit to the number of times you can be VP.

0.00% chance of a Sanders/Biden ticket. They are ideological opposites. Adding Biden to a Bernie ticket would mean a guaranteed loss of all/most of Bernie’s support.

As far as I know there is no term limit on VP.

I had hoped we would not have to find out that it could get as low as it did.

US elections are not about how terrible the candidates are. They’re about popular attention and conversations, and which “side” gets more votes – the specific choice of terrible corrupt-corporate big party figurehead is unfortunately not much of a limiting factor.

Even in the online conversations that aren’t even about a choice of candidates, but just approval or disapproval about the most atrocious POTUS in memory, you’ll see that people support their “side” even if they think the figurehead/POTUS is terrible in various ways.

Practically every voter thinks most of the candidates on both sides are terrible corrupt tools, but they still vote for them because of our disgustingly-stupid-yet-almost-no-one-pays-that-any-attention voting system, where we get ONE VOTE – “FOR” ONE candidate, and we’re told third-party candidates are wasted votes. So with two corrupt parties, it comes down to which flavor of fake policies you dislike least.

I don’t think Sanders would ever run with Biden. Certainly the progressives would tend to be disgusted with it.

And no way Biden would be VEEP again anyway. Been there, done that.