I would imagine there must have been a few…suppose the following scenario:
-you are a soldier in Vietnam-you get tired of fighting, and desert your unit
-you make it to Thailand or Cambodia-once thier, you assume a new identity
-you get a false passport, birth certificate, etc.
-you re-enter the USA, and settle down
Yeras later, you decide to “come clean”
What would ahppen to you?
Yes. It was me, and then I returned to the US and ran for Congress on the basis that I’m a war hero… wait… no. That wasn’t me.
More to the OP, Wiki claims that following the Paris Peace Accords in '73, 591 US combatants were returned.
One of the things that is tricky (in my mind) is that MIA can mean a lot of thing. As you mentioned, they can be deserters, and as such we can’t really pin their disappearance on the “enemy” - they’re just folks who chose to disappear and had the opportunity.
Perhaps your question is more like the “could we send in John Rambo and liberate these guys who have been held in captivity for 40 years?” If that’s the case, I doubt it - the Vietnamese government would have nothing to gain from that (at least to my thinking - Operation Hastings will begin in 72 hours is no longer valuable intelligence…
Apparently a few have. This site is by someone who worked for the government on the POW/MIA issue. It says that several MIAs showed up in the states later, but the ones he lists all seem to have been listed as deserters, except for Mateo Sabog, who was listed as MIA. So, the site says a few showed up later, but only proves one, but the guy is writing largely from memory of his years working on the issue.
There have been numerous investigations relating to POW/MIAs over the years. From reading that site, I get the feeling that they were able to cull the deserters out of the MIA lists, thus formerly missing people who turn up alive are most likely listed as deserters, which isn’t as newsworthy.
ETA: googling vietnam deserter found shows that several have been caught in recent years
But US border stations still sport MIA flags.
Still plowing the old Mia Farrow?
No statute of limitations?
There is a book by a Pvt. Garwood who was captured by the Viet Cong and held prisoner for a number of years. He learned to speak the language and became familiar with the culture and customs and pretty much became one of them. Other captured Americans claim he was a deserter and when he “escaped” was placed on trial for treason. I believe he was either found innocent or convicted of lesser charges. I can’t recall the title of the book but I think it was something like “Consorting with the Enemy” or “Conspiring with the Enemy” or something along those lines. Try Wiki or Google.