You know, I'm beginning to think that this may be a Northern accent thing. I speak with a Southern dialect, and it seems to be a reoccurring issue in the comments. Ever listen to Vietnamese songs or even your parents sing karaoke (totally my mom)? You know how Canadians, Americans, Britains, and Australians speak with different accents but they sorta sing with the same accent? I really think "một" is pronounced like that because that's how the Northerners say it. Just a guess though! Figured I could give my opinion/observation haha!
So I also live in Hanoi and speak the northern accent. While I agree that when people normally say một it doesn't sound like an 'n,' it would make sense for it to sort of sound similar. Think about how your tongue is placed in the same position for both an 'n' and a 't' so the main difference is really aspiration. Since ending consonsants in viet are cut short and less aspirated the 't' sounds like an 'n' in northern. In southern, however, the ending consonsant makes a 'p' in this case, which has an entirely different mouth shape. Granted, this guy's accent is still peculiar but I hope this helps people understand a bit about the reasoning for the pronunciation.