Very difficult to answer. Vietnam would have been under Hanoi's control by 1966 or 1967. LBJ could have accomplished even more domestically, but he may be blamed for being weak in communism. Like Truman was blamed for the loss of China. I'm not sure though whether he would have run again 1968 since Lady Bird was pressing him to retire for health reasons.
However, the Domino Theory was fully nonsense. Communism would have not spread further in Asia because of Vietnam.
But obviously he should have done so. Vietnam was his biggest mistake. If we could just cancel it out, he'd rank just below FDR and Lincoln among scholars.
The most ironic thing about the "Domino Theory" is that communism did spread into Laos and Cambodia
because of the U.S. intervention, not a lack of thereof. But without the hindsight LBJ might've received a lot of heat for being "weak on communism" indeed. Kind of Catch-22 situation.
As of 1968 it would take either total political collapse, like it happened in Rl due to Vietnam, or another heart attack while in office to prevent LBJ from seeking second full term. There's no way he would've passed on this otherwise.
