Since this is in the English section, I guess you're asking how the average English-speaker would pronounce these names. :) It's unusual for us to start any word with an "ng" sound, so we use the closest pronunciation we know. Ng = "ing". Yes, we would add a short "i" sound there. Please don't cringe. Nguyen = "NWEE-yen". A good speaker (in general) would try to learn the proper pronunciation, as Kraut9 and Eliot have suggested above.

Yeah it is Vietnamese. Pronounced: ngween

Ng is a Cantonese name. It is basically a N sound that you form in your throat, a bit like the N in 'Ring', with a silent G. Do not confuse it with the initial N from words like Noun, New. Nguyen is a Vietnamese name, I am not sure about that.