Actually, it's unclear. It would be covered by the12th,and clarified by the 22nd and 25th Amendments, as the initial Constitutional provisions (12th) were unclear as well, however the 22nd Amendment, which governs term-limits for the Presidency, only says that no one may be elected more than twice, or serve more than a certain number of years having succeeded the President as V.P. It's not clear (and endlessly argued by some) whether or not the language limits someone who has sat as President for two terms from sitting as V.P., as they would not be "elected" if they were to succeed the sitting President due to incapacity or death. :
The 12th Amendment, which covers eligibility for V.P., only shows the same criteria as President: a natural citizen, with 12 year or more residency, at least 35 years of age.It's unclear whether it excludes someone who has been a two-term president from being vice president, as that person would not be "elected" to succeed-though it does use the language "Constitutionally ineligible to be President." It doesn't say "Constitutionally inelegible to be elected President," which a two-term President clearly is, under the 22nd Amendment. Unfortunately, it's still a somewhat muddy possibility, with the language used, that a two-term President could be elected Vice-President, and succeed or act in the place of an elected President.
In any case, it's unlikely to happen just because of the arguments that would likely arise from it. Just not a politically savvy thing to do.....she's much more likely to tag Obama.