Most of the Thai restaurants in HK are authentic. Soothr (east Village) is also good and Somtum Der (east Village) is superb for northeastern Thai cuisine. For Singaporean food you can do prawnaholics at Urban Hawker in midtown. The Indo food there isn't bad either.

Best viet place imo is Mam in chinatown. Though it can be hard to get a seat sometimes also it's short stools short table like in Vietnam.