How long does it take by subway from the Vatican to Termini Station?
Unfortunately, there is no metro stop right at the Vatican. You have to walk north along Via di Porta Angelica/Via Ottaviano (right out the north exit from the Piazza San Pietro) to the Ottaviano metro stop (10-15 minutes...watch out, there are some great little clothing shops along the way!) then take the A line into Termini. Once you catch the metro at Ottaviano, it should be a 15 minute run to Termini. So, maybe half an hour, total.
There are several stops between, but they are quick stops so maybe 10-15 minutes to get to the termini rode. We took it real often to get around Rome and I don't ever remember it being a long ride.
Yes, I would say count on half an hour between the walk and the subway ride.