Florence to Vernazza

We haven't paid for a car yet, but I only have a four day train pass and am already using it all four days. I also was wondering if the train stopped a lot making it for a longer day traveling by train then car, but less stressful because you can relax and not get lost. So I am trying to weigh the options and see what is best.

A bigger problem may be the time involved if this is to be a day trip. It'll take at least three hours by train, although the fares are cheap. Even by car will probably take at least 2.5 hours, and you'll probably find that trying to rent a car for one day will cost you an arm and a leg.
(Add) In Florence itself, you also run the risk of running afoul of the dreaded ZTL's, special zones covering most of the old city in which cars without permits aren't allowed...and the fine for violating the ZTLs is hefty.

4 hours there and 4 hours back. If you go in the summer, that would leave you about 7 hours of daylight hours to get a quick overview. Would I do it? No way, but YOU are going, not me. Have a great trip.

Grace: To give you the other side of the picture, many here over the years have had a different opinion than the remark made in the 1st post: "Italy is a peace of cake to drive in outside Rome."

Many experienced travelers in Italy use the rule of thumb: if trains go there, take the train. And trains do go from Florence to Vernazza.

But as others have said, you'll want to decide if the travel time is worth it to you. If you decide to do it, and you decide not to go by train, then carefully consider how to avoid the big fines (several hundred dollars) for accidentally violating the Florence ZTL's that blanket the historical center of the city.