Sunday Morning mass

Maybe, but probably not. If Mass gets out about 10:00, figure at least 45 minutes to get to Termini. It's possible, but I wouldn't count on it. If you took a taxi it could work.

If you walk from the Vatican to Termini, it is a good 45 minutes. There are buses and, of course, taxi. Whether you can check in at 11 depends solely on the BnB's check in policy.

Right dk,

A taxi would be good. Probably the fastest way and less hassle on that day.

Make sure you take the official, white city taxi. (This link is borrowed from another post) https://sites.google.com/site/sctappromeforcruisetravelers/contenuto-della-app/about-rome/using-taxis-in-rome

A bus is cheaper but a bit longer (30+ mins) than a taxi ride. Walk a few minutes from St. Peter's Basilica to Cavalleggeri/Fornaci. Take #64 toward Termini. (You must buy your bus ticket, 1,50€ at a Tabacchi shop, corner market or some gift shops.)

Validate it on the bus Buses do not sell tickets or make change.

Keep your valuables close to you when riding buses and trains (entering and exiting too)

At Termini, get your bearings to where your AirBnb is. Look for a landmark.

A celebration Mass at St. Peter's will be extremely memorable. I went to one last December. Beautiful. Unforgettable. St. Peter's is my favorite.

If you're storing luggage at Termini while at St. Peter's, be sure to allow time to retrieve it.

What if we take the metro? I may need to contact the Airbnb and request a 11:30 or noon

Figure a 15+ minute walk to the Ottaviano metro stop, a few minutes of wait time for the next train, a 10+ minute metro ride and a few minutes to exit the Termini metro station. Best case around 35 minutes, more likely about 40 minutes.

Dk,

As mentioned, the train is an option, but it's about a 15- to 20-minute walk from St Peter's. If you're a fast walker, it should be ok. It will be a nice walk for a Sunday morning if your BnB host can give you an extra half hour.

Ride is 6 stops to Termini. Ottaviano - is Line A of the Rome Metro and is the station nearest to Vatican City.

You may know this already - but you have to buy your ticket either at window service or self service.

Although, some quick espresso places, corner market, or even a pastry shop may sell them along the way. Due to your time constraints, best to go directly to the metro.

It's good to have some Euro on you anyway -and for the window service. But, they most likely take a bank card for a ticket.

I can't speak about the self-service machine. But, it should be easy to navigate. I went to window service because there were more people waiting for the machines.

If you're not familiar with the metro, of course just confirm with one of the guards you are going in the direction of Termini. There are the signs anyway.