If you are looking for a decent and quiet place for a late supper in Lan Kwai Fong, there are more than just restaurants in Soho or Tsui Wah. The Night Market, a Taiwanese restaurant on 11 Stanley Street, is open till 11pm. If you are vegan, try their stir-fried vermicelli (HK$82/US$10.50) and ask to hold the meat. It is especially delicious if you order a bubble tea to go with it. They also have a vegetable section where you can adapt the dishes to vegetarian or vegan versions.
I find it quite a hidden treasure to stay away from the mayhem in Lan Kwai Fong, Hong Kong.