want to do day trip, no one seems to do train, only car or van?
Take the train, 70 minutes, any worries unfounded. In any stations one should be aware if surroundings with valuables secured.
staying by Vatican area hotel which train station for the the fast train , I just go there purchase ticket and board? First time, by myself. thanks for your time and assistance.
High speed trains depart from Roma Termini and Roma Tiburtina. If you are staying close to a secondary station, put it as departure point on trenitalia.com/tcom-en and Napoli Centrale as the arrival one. The site will find an itinerary for you.
It seems you think you'll have to take a deserted train in the middle of the night: you'll take a train full of tourists, teens and noisy families in the early morning. That's all: it's Rome, not Kabul! Don't accept "help" by anyone, if a firm "NO!" isn't enough scream "Polizia" out of your lungs and all "helpers" will disappear in 3 seconds.
You can opt to purchase a ticket on a fast train on line in advance, but if you take a discounted fare, those have restrictions regarding changes to anything: day, time, train, etc. If you are sure what days and times you intend to travel and feel comfortable buying in advance, though, you might save on the fare that way. Plus you would already have a ticket with an assigned seat in a specific train car (carriage).
You can take a taxi to the train station you opt to use.