Here's what I think.
First, both arts are very different. BJJ (Brazilian Jiu Jitsu) focuses on ground grappling techniques, while wing Chun is all about stand up fighting.
Second, it's all about the individual. It's normal that there's always going to be someone better than you out there, and fighting is just the same. The more you train and understand your art, the more your probablility of winning or surviving will be increased.
Now, if both fighters, BJJ and Wing Chun are equally advanced and fit, then Wing Chun will win. Think about it. Wing Chun specialises in the first (hands), second (elbows), and third gate (shoulders) of fighting, while BJJ only focuses on taking someone to the ground. BJJ guys have very basic first and second gate fighting techniques. Most of their stuff is past the third gate, so you tell me how can a BJJ guy enter those first two gates of an advanced Wing Chun guy if they can't even compete at those distances. Their only way is to rush the WC guy below the waist or knees. Granted most people don't know what to do in those cases, but believe me, it's not hard to keep your stance if you've got good structure, footwork, and sensitivity. Mind you that I'm still talking about equal body types. When someone rushes low you should follow and sink/widen your stance. Don't widen your stance with one leg in front and the other in back, because you'll give away your front leg to the BJJ guy. Instead, lower your stance while applying forward/downward energy to the BJJ's neck area. Your legs should be behind you, out of reach from the BJJ guy, and you should be somewhat on top of him. This by default and a little guidance, bring his head near your elbows giving you access to a choke or a kill.
Through my experience in sparring, even though I'm no pro fighter, chasing center and chasing head is the best way to keep your attack/offense pointing at the right place. In most cases, people will generally want to tackle you. What has worked for me in the past is to always have your intensions or energy pointing towards the centerline. That doesn't mean touching the other dudes chest or face or anything, but the intension should be there. Once the guy sinks for a waist or below the waist attack, sink your stance and keep your arms near the person's head. It should be similar to a low Bong Sau and should come naturally. The result is a neck choke. So what if they manage to move your stance, if you have good structure, it would be hard to make you off balance. The bottom line is, where the head goes, the body goes. So if you have the guys head wrapped around your arm, then you have the advantage. You can choke or snap their necks at any sign of movement. Wing Chun guys should be good at that, given their ultra sensitive nature.
So there you go, just follow the head, and tighten your grip. Of course this might not work agains a pro UFC/BJJ guy who's twice your size because their reach and mass can overwhelm your structure. Just be smart about it break their rythm and balance. Always be one step ahead of them.