The apartment was perfect, it had everything you could need for a city break in Rome. It was only a short 15 minute walk from Termini station and had very good public transport links with a local tobacchi store selling tickets for €1.50 which last for 100 minutes after being stamped on the bus. You can reach Vatican City using the 71 and 64 buses, which will take roughly 30 minutes.
The local area has a lively, friendly vibe and we definitely felt safe with Antonella's apartment as our base. However, we would just avoid the main roads alongside Termini Station after dark as there is the usual homeless community around the station (as you would expect around a station in a major city).
All in all, we would 100% stay at this apartment again and recommend you stay here and Antonella is an amazing and thoughtful host.