Look below - similar questions ghave been asked and answered. You can get a trip to Vesuvius from Ercolano; the booking office is outside the station there. Ercolano is nearer Naples (on same railway line as Pompeii) and its ruins are smaller and more manageable than those at Pompeii.
I've done this trip a couple of times a few years ago. There is a bus that takes you to the base of Vesuvius but it's easier and better ( cheaper too if there are four of you) to take a taxi. Just a word of caution though Talk to several taxi drivers before committing yourself get prices and generally assess the drivers attitudes most of them speak some English. We ahd a great driver same one each time who not only treated us very fairly but got us a discount on the hire of suitable footwear and hefty sticks to help the climb.It's a pretty stiff walk when you get towards the top and you need proper boots for it Take plenty of water with you too hope this helps.