Do you mean a tour from Positano or a tour of the archaeological site only? To get to Herculaneum, google maps and viamichelin.com both show a trip of 1.25 hours at least by car. By public transportation, you'd have to take the bus to Sorrento and then the Circumvesuviana train to Ercolano and walk about 10-15 minutes downhill to the site (hence uphill on the return). There are several buses an hour but they start in Amalfi, so unless you leave quite early, you may not all be able to get on the same bus. It's about 40-50 minute trip, unless traffic slows it down. The train runs twice an hour and takes close to an hour. So I would only recommend going on your own by taxi, though you will probably need 2 taxis. I doubt you'll find a group tour that is only Herculaneum, especially from Positano. It's pretty easy to see the ruins on your own, with their audio guide. If you want a private tour, check the RS guidebook. They probably have guides listed, maybe even with a van.