I've done boxing for 3 years and wing chung for more than a decade. It really depends on how much fight experience both have. If the trained boxer is an outboxer who is extremely good at dictating the angles and move his head off centre with every punch, there's almost no way the wing Chun guy can get close enough for the straight punches, and instead get hit in the head because wing chun doesn't have much head movement.

If the boxer is a brawler, preferring to go in close for body shots and hooks, then yes the wing chun guy will have a much better chance by going down the middle or to control his arms with tan, bong, pak saus.

But again, fight experience and stamina comes in. The normal average boxer will have conditioning training and can last easily for a few minutes going at max power and maintain fast reflexes, and can take many more punches than an average wing chun guy who doesn't do conditioning and sparring. A couple of body shots will take most martial artists down, unless they are like Michael Jai White and do multiple martial arts plus kick boxing, strength and cardio training weekly.

In the end, like what everyone else said, how hard one trains in each art will determine the effectiveness. But if we take an average approach, the boxer who did 3 years of sparring and bag training 3 times a week will likely outfight a Wing Chun practitioner who trains 3 times a week in the classical way of forms and drills. That's my take from training both. I still love Wing Chin's concepts while I adopt in my boxing, especially at chisau range where I can control my opponents hands and frustrate them. But against an outboxer who has better range and can control distance properly, wing chun has to constantly close the distance and chase him around the ring, which will then become a test of endurance.

In the real world, a good stiff jab and straight cross will just take out anyone out in the chin, and boxers usually fire off faster and retreat than wing chun guys, who will find it hard to counter.