There is a rumor Nathan Phillips, who claimed to be a Marine and a Vietnam vet, could not be both. I looked him up and he was 64 at the time of the recent incident, so the very oldest he could be is just turned 65 this week, and thus born January of 1954. There is one report of him celebrating his birthday Feb 22. I remembered that the USMC left Vietnam just before I graduated high school in 1971, but memory is fickle, so I looked it up and yes, they left in May 1971. The oldest Nathan Phillips could be was 17 years and 4 months. 17-3 or less is more likely. You could join the USMC at 17 with parental consent, but then you would have Basic Training to go through. That is currently thirteen weeks. I don't know what it was then.
If it was seven weeks, then he would have to have gone to Basic on his birthday, and gone to VN immediately in mid-April, then brought back a month later. This seems...unlikely. Does anyone think the Marines would be sending new units over that they would be bringing back in five weeks? Sometimes just plain arithmetic can tell you things.
He might have lied about his age. That could change the picture. But even with that, initial deployment would be in 1970, and is an unlikely scenario. I believe Force Recon still had a presence in Vietnam after 1971, but I don't think he's made that claim. One would not be in Force Recon and then later describe oneself as an infantryman.
In some places he claims to have been a Vietnam vet, in others he says he was an infantryman in the Vietnam era. I suspect the latter is more likely true, as the "era" runs to 1975 for the nation, though not the USMC. Is that a common exaggeration, to falsely claim to have been deployed to a war zone, or more mildly, to say "Vietnam-era veteran" and not make the correction when people conclude you were in VN?