Yes, a former president can serve as vice president. The U.S. Constitution does not explicitly prohibit a former president from holding the office of vice president. However, there are some considerations to keep in mind.
1. **Constitutional Eligibility**: The 12th Amendment states that no person constitutionally ineligible to the office of president shall be eligible to that of vice president. Since a former president has already met the eligibility requirements for the presidency, they are also eligible to serve as vice president.
2. **Succession**: If a former president were to become vice president and the sitting president were to leave office (due to death, resignation, or removal), the former president would once again assume the presidency. This scenario would require the former president to be eligible for a third term, which is currently prohibited by the 22nd Amendment. However, the 22nd Amendment only limits the number of times a person can be elected to the presidency, not the number of terms they can serve. Therefore, if a former president served two terms and then became vice president, they could theoretically assume the presidency again, but only if they were not elected to it.
3. **Political Considerations**: While it is constitutionally possible, it is politically unusual. The role of vice president is typically seen as a stepping stone to the presidency, and having a former president in that role could create unique dynamics within an administration.
In summary, while it is legally possible for a former president to serve as vice president, there are practical and political factors that make it an uncommon scenario.