Pompeii vs. herculaneum

What is the rest of your itinerary? Perhaps you could add it earlier. MUCH depends on personal interests.
Pompeii is much larger (and it's Pompeii, with all that entails). I would consider Herculaneum as an add-on for someone with particular interest or with limited time.
I did not find Vesuvius as important because you see it from everywhere--I just did not feel compelled to climb it.
I was fascinated by Pompeii my whole life and spent a large part of a day there. I would not have wanted to get all sweaty and tired and then go on a train, but that is just a personal issue.

We did Pompeii, Herculaneum, and Villa Oplontis, but not all on the same day, and not all with a car and driver. Because you are staying in Positano, an expensive car and driver is the best way to see any of these sites. (Note that the Frecciarossa train tickets are not flexible, they're for a specific departure only.) We had no interest in Vesuvius. My OPINION is that Vesuvius is a waste of time, and I wouldn't go anywhere near it on a hot day. Given your schedule, I would not recommend doing both Herc and Pomp on the same day. But personal preference is the controlling factor. Do you care what time you get to Rome?

I personally found Pompeii more iconic and more frequently referred to in art history literature. It's bigger, and hotter (Month of the year, please) and takes 2 to 4 hours for a reasonable visit. Have you scheduled plans for lunch? But there is nothing wrong with Herculaneum. Why did you select it? (One good reason is that you won't have to walk back up the hill to the Circumvesuviana stop, since you have a driver.) Neither site is a complete city, since much remains buried under modern Pompeii and modern Herculaneum. But Herculaneum is much smaller and has slightly more intact wood fragments.

I mention lunch because there is no food inside Herculaneum (and you CANNOT re-enter on the day's ticket.) You can eat faster in Pompeii's touristy hot cafe than you can at a (nicer) seated restaurant (i.e. with "slow food") in Herculaneum-town. Both have good baggage checks, but you don't need that with the driver watching your stuff. In both ancient cities, you have no control over which Villas happen to be closed for conservation or due to lack of guards, on any given day.

Edit: Pompeii's cafe has hot food available! The building is blessedly air-conditioned.

I've been to all three and if I had to make a decision I'd say drop Vesuvius. You can see it from everywhere and the crater is certainly unimpressive, it's just a grass filled crater. The end result isn't worth the hassle (you don't say what time of year you'll be going).

If I had to choose I would say that Herculaneum was my favourite but that would be by a very thin margin and based primarily on the size and general lack of crowds that Pompeii experiences however Pompeii is Pompeii and certainly shouldn't be overlooked if you're there. My advice, forget Vesuvius, get to Pompeii early, have a late lunch and then visit Herculaneum. It'll be a long but fascinating day.

Agree with JC. Skip Mt. Vesuvius and do both Herculaneum and Pompeii. They are totally different and each is worth of several hours.

We did Mt. Vesuvius and Pompeii and really enjoyed both. I think whether you prefer Herculaneum and Pompeii to Vesuvius depends on your interests. I thought it was a beautiful hike up the mountain and very different from visiting ruins. The views were wonderful. We did the two different days and I think Pompeii and Mt Vesuvius in one day would be challenging. I do not think you can do all three.

Beth