Having seen some of the photos of the Embassy and the helicopters being dumped off aircraft carriers, I wondered - were there and G.I.'s that didn't make it and had been left behind and is there any account of what happened to them afterwards?
The short answer: no there was never any evidence to suggest that anyone was left in Vietnam.
The long answer: The US did a very thorough investigation through a Senate Select Committee on Vietnam POW/MIA soldiers beginning in 1991 after the concept of a POW being left in Vietnam was embraced in pop culture. There is a famous Newsweek cover from 1991 that purported to show 3 American POWs that were left behind, but was later proven false. The committee was chaired by John Kerry, and held hundreds of hearings trying to turn over evidence to account for missing POWs, ultimately never finding evidence of a single POW being left behind.
I am currently on my phone and unable to link to sources. Hopefully more qualified people will be able to cites some great primary sources for you to take a look at.