Theresa,
I vaguely recall that there's a small rail ticket office in Vernazza, but there will also be a Kiosk. If you happen to be visiting Monterosso, that has a full service (larger) ticket office and you can buy whatever tickets you need there.
Regarding validating tickets, you can either check the description of each train on the Trenitalia website or look at the letter preceding the train number. If the letter is "R" or "RV", then validate the ticket prior to boarding the train on the day of travel. DO NOT validate before use, as they have a "shelf life" once validated of about six hours (depending on route length). The validating machines in Monterosso are down in the transfer tunnel (sottopassagio) as I recall. Rail tickets in Italy are normally printed on light card stock about 3.25" x 8". In the C.T. you may be issued the smaller theatre-style tickets, and those are inserted into the machines on the left side.
If the train letter is "IC", "ES" or "EC", those trains have compulsory reservations, which are specific to train, date and departure time, and you'll have an assigned Car (Carrozza) and assigned seats (Posti). The tickets can ONLY be used on the one train listed on the ticket, and don't have to be validated.
It's awkward to purchase tickets in advance for the Regionale trains, and pre-purchased tickets come with restrictions. There's absolutely no reason to buy those in advance, as you can easily buy them locally.
Staying in Vernazza will not be much different than staying in the other towns of the C.T., in terms of the trip to Venice. There's a departure at 09:28, arriving Venezia S. Lucia at 14:20 (time 4H:52M, 3 changes at La Spezia, Pisa and Florence). The first and third legs are via Regionale (VALIDATE) and the second and last are via InterCtiy and Freccia, respectively (compulsory reservations with assigned seats).