You might want to take it in to a local cycle shop to have them look it over. They could probably tell a lot about the health of the engine by how it's running. You could also get a ballpark number about an engine rebuild. That way you can weigh the cost of the purchase and the worst case scenario (engine rebuild at some point down the road).
That seems like a lot of miles for the age so I'd guess it wasn't babied, but a new one like that would probably be $6k to $8k. There might be a forum for Vinson owners, check what they think about the longevity and reliability.
What are you wanting the ATV for, utility or sport? I've got a Kawasaki Bayou 250 2x4 with a custom box that I absolutely love for chores around my property and something new like that would be in the $3k-$4k range. I've only gotten it stuck twice and that was definitely my fault! It's not in my budget to get a larger one but I haven't missed it! You might want to weigh buying used vs a small new one with a warranty.
Hope that helps!
