Can an ex US President take on a Vice Presidential role?

I am not aware of any US laws regarding an ex president being vice president.

But do we really want Bush back as VP or maybe Jimmy Carter?

I googled "bill clinton vp possible?" and found several sites that say he can, or any other former President.

It would cause a few issues in that they could not step in and be president if the President was unable to function, during times of surgary, or if killed.

Bill Clinton or any other 2 term former President cannot hold a position that put them in the line of succession to the Presidency. Since the VP is the first person in the line ,and Clinton has served 2 terms then he is not eligible . Carter and GHW Bush however are .

I'm sure that if either GW Bush or Bill Clinton were to hold any of those positions that there would be an immediate court challenge to their eligibility. The only gray area in this is the proposition that the two term restriction is two elected terms. So perhaps that technicality would come into play... but I doubt it.

Can an ex US President take on a Vice Presidential role?
It depends whether the persona in question has been President for 1 or 2 terms. Because it is possible that the VP can become President if the sitting president becomes incapacitated, and because it is Constitutionally prohibited for someone to serve more than 2 terms as president, it would be illegal for a former president who has served more than one term to become VP. However, if he has served 1 term or less, then it would be legal to do so.

Elliot