Though belonging to China, Hong Kong is much more western in culture and habbits, and this is certainly convenient for the travelling vegetarian as well. I have last visited the place after Wikimania in summer 2007.
My stay was short and so are my hints below.
Vegetarianism in popular culture
China is not a country of vegetarians, with the possible exception of strict Buddhists.
However, I found that some Hong Kong restaurants even feature handy green leaf symbols to indicate vegetarian dishes, hence I assume that the concept is not so uncommon in the city. Thus, if you are willing to look around a little, you will most certainly find some restaurant that is convenient for vegetarians and non-vegetarians alike. Of course, there is still a difference between dishes without meat and dishes that are really vegetarian (no meat gravy etc.) – if you are concerned about details, you better look for some further hints online.
There are also many rather more western snack bars or fast-food restaurants, that offer familiar looking food including some vegetarian options.
English is certainly more common in Hong Kong than in any other place in China. For longer stays, it still might be useful to compile a list of essential phrases similar to the ones I made for Taiwan. Note, however, that the most common form of Chinese spoken in Hong Kong is Cantonese, written in simplified letters. Hence my (traditional Mandarin) phrase list for Taiwan is of little use.