I disagree that a person with one discipline could not dominate in MMA.
When it comes to Wing Chun there is some controversy about Traditional (If there is such a thing) or "Modified" and which is the "Real" version. I bring this up because there is different disciplines that have developed.
As we know Wing Chun works on the principles of the least work to get the job done. Bruce Lee talked about how there was a lack of proper "Fighting Stamina" in the style.
The reason I believe that a person knowing Wing Chun alone could be successful is:
1.) Efficiency of the style
2.) Trained to fight based on touch, not site
3.) Bil Jee Form - This form not only involves the deadly killing and maiming technics but also technics to get out of situations were you have been taking off center or to the ground. (Bone Breaking technics etc.)
4.) My final reason is the substance of the short punch. Wing Chun trains bone breaking power at a short few inch's. Often times when a person goes down to a grappler there is moments if he were trained he could drop a short punch that would devastate his opponent.
Anyway opinions vary and until we see a Wing Chun master in UFC we will never know how well they would do.