Personal recommendation: allocate most of a day to Pompei, then another day to Vesuvius (a.m.) and Herculaneum (p.m.).

I agree with others re. Herculaneum being better preserved. Also easier/faster more compact to navigate. It is COOLER due to shaded areas by the sea. No need for a guide. You should consider seeing the Naples Archeological Museum everything from both sites is housed there. In extreme heat a hike up Vesuvio after hours in the hot sun is quite daunting. Not a good time of year for that.
Public transport is the best way to travel to these sights.
A good alternative to Pompeii is Ostia Antica 1 hour outside of Rome.
Enjoy!
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