Agreed, the ring gear on the Vinson also sits on the opposite side of the pinion, the rear ends are completely different. Bike Bandit is a good place to find OEM parts but they usually don't have the best prices, like Gearhead said, online dealers are much cheaper than dealerships for the same factory parts. I personally like Babbits Online Parts and Service Honda, they have the best prices I have seen. Keep in mind, there are a few different models of the Quadmaster 500 that were offered in 2000, make sure you check your VIN# to make sure you order parts for the right one.
A rear gear set for your machine from - Bike Bandit - $612
Babbits - $540
Service Honda - $500
Yours truly, sincerelly, love always, Josh aka The DirtDemon.