Sara, regarding Vesuvius, I wouldn't attempt the climb if you have limited mobility. A driver can only take you so far; you will need to get to the top and back under your own steam. It's a steep uphill (and then downhill) trek on a loose, dusty, lava-rock surface that has reportedly presented a challenge to visitors who aren't especially limber or are unused to that sort of activity.
You can see the cone very clearly looming over Pompeii archeological park when you're there. I'd keep the volcano experience to just that.
Regarding Pompeii: it's also going to present some mobility challenges as it involves a lot of walking on uneven surfaces with few places to sit down. You can hire a guide at the scavi but given your mobility challenges, you might want to hire one in advance who is aware of your disability and prepared to move at your speed? You will cover just a fraction of what is a vast excavation in a two hour tour.
I've not yet been to Herculaneum but Sage Travel ("The European Disabled Travel Experts" and endorsed by Mr. Steves) rates it pretty highly - 4 stars - for accessibility, while they rate Pompeii only 2 stars. While that rating is mainly for visitors using wheelchairs, the same ramps and whatnot that benefit them will also benefit you!
https://www.sagetraveling.com/wheelchair-friendly-roman-ruins-herculaneum
https://www.sagetraveling.com/wheelchair-access-at-pompeii-italy
All said, I'd scrap Vesuvius entirely and go to Herculaneum one day, and Pompeii on another. In addition, you could go back to Naples after Herculaneum and see the very highly rated archeological museum, where many of the treasures from both excavations reside. Too tired to put one foot in front of another? Their visitor info states that wheelchairs are available for borrowing with proof of disability; you could do your exploring sitting down. :O)
https://mann-napoli.it/en/home-english/
Oh, and I've seen this driver recommended on the forum by a poster I trust:
https://www.topexcursionsorrento.com/?portal_referer_id=46