Making a Vietnam game is a small taboo in the US you have to remember this war was lost and the Veterans faced a lot of hate after their return. There is a reason why the best Vietnam game so far (Vietcong) was not made by a US company (they are Czechs). A Vietnam game wouldn't sell to well. Battlefield Vietnam didn't do so well afaik.
Supply and demand run the markets.
Just to explain, a Vietnam game would not be taboo in the U.S.; frankly, there are a lot of people who disagree with the strategy that was used in the war (like not invading the North, because of concerns about Red China).
However, because of Afghanistan and Iraq, the Vietnam War is no longer really discussed in popular American culture.
It seems that about roughly 10-20 years after an event, that is when it receives the most historical scrutiny and nostalgia (the TV sit com Hogan's Heroes was made in the 1960s; Happy Days and M*A*S*H, situation comedies about the 1950s and Korean War, were made in the early 1970s; etc.).
So, in the 1980s, there were all these Rambo movies (there was even a Rambo cartoon) and Chuck Norris films about MIAs, and films and some U.S. TV series / shows that discussed Vietnam (most famously PBS's "Vietnam: A Television History" based on the book by journalist Stanley Karnow) or whose fictional characters portrayed Vietnam vets.
It is just that Afghanistan and Iraq are now the focus of military interest (although I would say, nationally speaking, WWII still is at the top of everything) and have overshadowed Vietnam.
Then, I would say from a national perspective, (not necessarily from a video game perspective), the historical conflicts most Americans are interested in, after WWII, would be the American Civil War, then Vietnam, then the Revolutionary War, Korean War, The Mexican War / War of 1812, and I would say WWI comes in last (the TV networks are showing documentaries and some movies about WWII this week for Veterans Day, but absolutely nothing about WWI [at least in my area]).
This past January was the 40th Anniversary of the TET offensive, but I didn't see any history programs about the Vietnam War.
There were some shows that dealt more broadly with the 1960s, but those seemed to be more geared to nostalgic Baby Boomers, and they didn't focus on Vietnam.