djevans,
First regarding your question on why the prices on Trenitalia may be higher than Italo. If you're searching for trips in the near future, it's likely that the heavily discounted tickets on Trenitalia are completely sold-out, and therefore only Base Fare will be displayed. If you're planning to pre-purchase tickets for the premium trains, you can obtain the cheapest Super Economy or Economy fares by purchasing well in advance. However, keep in mind that will lock you into travelling on a specific train, date and departure time. Tickets are NOT interchangeable with other trains, and if you're caught with an invalid ticket for the train you're riding on you'll face hefty fines which will be collected on the spot! You may find it helpful to have a look at the excellent Ron In Rome website for more information.
Here's a brief summary of your options on the trips you mentioned.....
- Rome to Florence - travel either via Freccia or Italo high speed trains. Note that Italo uses Roma Tiburtina rather than Termini, so you'll have to depart from a different station. They both use Firenze SMN, so no difference there. The trip will be about 90 minutes at up to 300 kmH. You can buy tickets in advance and perhaps save money (if you buy early enough) or buy at the station from a Kiosk or ticket agent. If you buy just before the trip, you'll most likely be paying Base Fare.
- Florence to Vernazza - this is a very easy and common route, but will involve a couple of changes. The trains will likely be a combination of premium trains (Freccia, InterCity) as well as Regionale, depending on which particular departure you choose. You can save by purchasing the premium tickets in advance, but again that will lock you into a specific train, date and departure time. If any legs of the trip will be via Regionale, don't bother trying to purchase tickets in advance as there's no difference in price. It's just as easy to buy those at the station, preferably from a Kiosk as the ticket office at Firenze SMN usually has a HUGE queue! It's VERY IMPORTANT to remember that with Regionale tickets, you MUST validate these prior to boarding the train on the day of travel, as those with unvalidated tickets will again face hefty fines collected on the spot! DON'T validate them on the day prior to travel, as they have a "shelf life" once validated and will have expired by the time you use them. The train I'd likely use on that route departs Firenze SMN at 09:28, arriving Vernazza at 12:19 (time 2H:51M, two changes in Pisa and La Spezia, trains Regionale Veloce, FrecciaBianca and Regionale).
- Vernazza to Venice - again a very easy trip, but will involve several changes. The trip I'd use on that route is a departure from Vernazza at 09:28, arriving Venezia S. Lucia at 14:20 (time 4H:52M, three changes at La Spezia, Pisa and Firenze, trains Regionale Veloce, InterCity, Regionale, FrecciaArgento). That's about the shortest travel time you'll find on that route, and those trains will get you to Venice at a decent hour (for one thing, your room in Venice should have been serviced by then, and there's still time to do some exploring before dinner).
NOTE that when researching these routes, I used an arbitrary date of 15 May, so the schedules may change slightly in September. Use the Trenitalia or bahn.de websites to check these closer to your travel dates. New schedules will be uploaded about mid-June.
You can use either Trenitalia or Italo on the Rome-Florence and Florence-Venice trips, but the other trips will all be via Trenitalia.
Happy travels!
