There is another possible explanation.
The “w” sound of the glide following the initial “ng” is described phonetically as a labio-velar approximant. This means that in pronouncing “w”, an incomplete closure is formed both at the lips (labial) and between the tongue and soft palate (or velum).
Since the velar nasal “ng” is also pronounced at the velum, it’s likely that for some Vietnamese speakers, there’s an anticipatory sound change going on. Instead of fully articulating the “ng” with a complete closure between tongue and velum, some speakers may begin to form the “w” glide prematurely – all while the velum is lowered and (a little) air is escaping through the nose. Essentially, these speakers are pronouncing a nasalized labio-velar approximant at the beginning of “Nguyen”, instead of a velar nasal followed by a labio-velar approximant.
Such changes can be found among individual speakers of any language. Most of the time, it amounts to nothing and can be regarded as a linguistic quirk of an individual or family. If enough speakers share a battery of such changes, however, they can become marks of a separate dialect.