I hate to quibble, but America was not defeated in Vietnam, at least not militarily. United States forces never lost a major engagement to the Communists.
Morever, when South Vietnam did fall, it was not to a "local fighting force" (the Viet Cong were pretty much eliminated as an effective force after the Tet offensive). Instead, it took a full-scale invasion from North Vietnam to defeat the South. That invasion would have failed had the United States supported South Vietnam.
The Vietnam War was lost in the halls of Congress.
Yeah. Either that, or you could say it was lost on the nightly TV network news broadcast. I'm glad those days are behind us.