The US will lose regardless. Vietnam was an unwinnable war.
What? Really?
a) Tet is seen as reality, massive North Vietnamese defeat. LBJ wins second term, crushes North Vietnamese. Then withdraws troops and backs South Vietnamese only with airpower (as historically: South Vietnamese + US airpower defeat North Vietnamese).
North Vietnamese run low on conventional supplies, and after a '75-76 failed invasion (as the… IIRC '73 invasion went) the USSR turns their attention to Afghanistan. North Vietnam then lacks the support for a further offensive.
b) US adopts proper counterinsurgency tactics early (retire Westmoreland, Abrams gets command). Viet Cong wiped out earlier then OTL Tet in '68. North Vietnamese conventional forces defeated, as usually happened.
c) No Watergate. Nixon continues supporting South Vietnam as per OTL and the '75-76 offensive is defeated as the earlier one was. See version a).
Etc….
The Vietnam War was winnable. It was a bad idea to start, and it might be a bad idea to win, but it's certainly winnable (winnable = Korean type situation).