The American Bicentennial is one of my earliest political memories, and I don't know if my ignorance of the impact the Fall Of Saigon had on the mood is because I was too young to sense it, or if it's just that there WAS no impact, April 1975 being a mega-eternity away from July 1976.
My GUESS is that, unless American soldiers have somehow been sent back into combat by July '76, there isn't much impact on the party atmosphere. The impression I've always gotten is that, once the US withdrew its troops, the average American didn't see his country as having a dog in the fight.
There might be a bit of sombre juxtapositioning in certain media quarters("As we witness the loss of freedom in South Vietnam, we celebrate its historical triumph here in America"), but nothing that would seriously get under the nation's collective skin.
And it probably doesn't hurt Ford in the election, since the kind of person who was still cheering for South Vietnam post-withdrawl likely isn't going to abandon the GOP.