This idea has been tossed around for the past 15 or 20 years. To the best of my knowledge, it was first suggested by Wing Chun instructor Karl Godwin, in his brilliant article, "In Search of Wing Chun's Roots--Did it Evolve from Western Boxing or the Shaolin Temple?", which appeared in the June 1986 issue of Black Belt magazine.
Godwin noted the following:
1. There is no actual record of the Buddhist nun Ng Mui even existing. Godwin said that Master Leung Ting stated that he "doubted the historical events and the existence of Wing Chun characters before Leung Jan."
2. Every Wing Chun master has a different location regarding the various Wing Chun temples.
3. Wing Chun masters are totally inconsistent regarding the art's age, putting it at anywhere between 150-300 years old.
4. Wing Chun lacks the ritual of other Chinese styles. There are no salutations at the beginning of its forms.
5. Wing Chun is technically quite different from other Chinese styles.
6. "The pacifist-type tradition found in most kung fu systems is practically nonexistent in Wing Chun".
7. Wing Chun takes much less time to become proficient in, when compared with other Chinese methods.
8. No weapons are native to the Wing Chun system. The bart cham dao and long pole were "introduced from another style".
I already outlined the British "vertical fist" punch, which seems very Wing Chun-like.
Karl Godwin is not a Martial arts historian and to be frank, it seems like that he has rarely practiced other traditional Chinese MA. I've just done some research on the development of southern Chinese martial art and found all of his evidence to be very weak. A lot of
southern styles have vertical fists and chain punching along with with parrying guards. Just take Hung Ga for example. It goes all the way back into the early Qing, the Yong Chun White Crane dates back to the late 17th century. All of which are at least as old as British bare knuckled pugilism. No serious martial art historian today would consider Wing chun to be related to boxing, and if you've seen wingchun, its not that hard to realize that its far closer in philosophy to traditional Chinese boxing. The force generation in wingchun is traditional chinese ma based, focusing on the lower part of your body to generate power and stable stance with a firm root on the foot. I'm not exactly sure whether Wingchun is more "pacifist", I haven't trained in wingchun for any length of time, the only southern martial art that I've came across often is the black tiger and Hung ga, both have vertical fists, the former is full of them; but wingchun's fighting philosophy is really little different, it tries to keep its stance at all times and avoid positions where you are clinched to the opponent where the stances are thrown off. This is very different from boxing where the upper half of the body is more focused. Wingchun's sticky hand practice is also really similar to the other Chinese styles, so other than some superficial similarity and a completely undocumented relation, its groundless to assume that it came from boxing.