This thread does not give the month of the trip. Heat, Sun, and Crowding are factors at Pompeii and Herculaneum. Is this your first trip to Europe? It makes no sense to visit (and taste at) a winery while already short on sleep!
You would be right if you think that Positano is not a good location for daytrips. It is a very different environment than Sorrento, so your desire to stay there may (or may not, I for one don't care for smaller, pebbled beaches) be justified.
Assuming you can easily afford the car service, you might think about the amount of money you'll be paying for the car to sit and wait for you. Could you use the same dollars in a better way by using a second car service another day? BTW, if you are going to your next destination from Positano, you need to plan the extensive travel and time necessary for your departure day. That may clarify your total transportation needs.