Is a fever of 101 concerning in a 2 month old infant in the 24 hours following vaccination?

Fever. fever more than 24 hours after vaccination should have a different reason Make sure no other issues like breathing problem, vomiting, etc.
Fever after shots. Fever can be normal after shots. Follow the link for a helpful list of common reactions as well as things to be concerned about and when to call your pediatrician. It is very informative and complete. Good luck!