KC, great that you can consider purchasing early. You can get some great discounts but they are non-refundable. You don't indicate when you intend to travel but you can buy tickets on www.trenitalia.com up to 119 days in advance. It's really 120 days but often times the 120th day doesn't show. The only difficulty is if mid-June or mid-December is inside the 119 day window. These are update periods for the Trenitalia schedules and you most often cannot see many of the runs beyond those dates.
Roma-Termini to Vernazza is quite easy. Two routings. One via Florence, Pisa and La Spezia. The other is via the western part of Italy (Civitavecchia, Pisa and La Spezia). The routing via Florence is more expensive as you will be on the very fast trains from Rome to Florence. You will change trains at La Spezia Centrale no matter which routing you choose. This last train will be a Regionale train.
Yes you can buy the entire run in advance on www.trenitalia.com. Normally, you cannot purchase Regionale tickets online more than 7 days out from the time of travel. However, when combined with a fast train, you can. And, you will have the seat reservations on the fast train and the ticket for the Regionale train will be pre-validated that will have to be used within 4 hours. You will receive all tickets within a couple of minutes after purchase. You can print out at home. For the fast train, you will see a 6-character (numbers, letters) PNR code. This is your ticket. The conductor carries a wireless PDA that he uses to enter the code into the train system. You're good to go. For the Regionale ticket, you will not see a PNR code but you will see an 8-digit code. The ticket will say that it is pre-validated.
Here's a screenshot of a partial schedule for Dec 10. Notice that I did not go past mid-December. Note the starting fares. Not bad. http://gyazo.com/da717cf3b6d31d9cfd781c7ef27743bd