Now when it comes to style versus style questions I aways like to point out that it really depends on the preparedness of the individual, not the style. However, since your question was not Wing Chun vs MMA fighter, it was Wing Chun Fighter vs UFC Fighter - I have to appreciate the level of dedication and commitment an MMA fighter has to reach to be in the UFC. Therefore, as it was stated about the UFC fighter. By time someone reaches that level of competition, even if they are only 22 or so, they are absolutely a great martial artist.
Wing Chun, they train in weapons and last i checked an armed oppenant usually defeats an unarmed one. Most of what you see in the UFC will get you killed against an armed oppanent. A double leg takeone doesnt work the same against someone with a knife. When I was a kid about 17 years ago a fight broke out at a park we use to play basketball at. One kid wrestled the other to the ground. The other kid had stabbed him a few times on the way down broke free and ran. Luckily the kid lived. I myself was stabbed in a similar situation. That is always in the back of my mind. By the way I wrestled in jr high and high school and did judo in the army and through college. So I am not biased against grappling, but it does have its limitations.