To find ALL military aircraft in Vietnam during the war, I searched for "Military Only", "Vietnam" under the location field, and sorted the results by date (oldest first).
Here are the results of that search. It brings up a few US Army and US Marines aircraft that didn't come up in the previous search.
Despite what some others might think, photos of US Military aircraft in Vietnam during the war are indeed quite rare in our database. Indeed, as you can see in the above search results, there are only about 10 or so.