It's Win, with a change in tone when you pronounce it. There's no alternate. The other pronunciations are for others to correlate the name Nguyen with your pronunciation, so when I say my name, it's nu-win, but it's properly pronouced Win. The Vietnamese language is monosyllabolic, meaning no words with more than 1 syllable....
The ones who pronouce it incorrectly are either doing it, because it's what Americans know or they're 2 or 3rd generation Vietnamese and prob don't even know any words in the Vietnamese language.