Assuminng Kennedy lives and wins a second term, Johnson does not win the 1968 nomination. Johnson would have had little support from Kenendy, who disliked him, and Vietnam probably would've divided the country in the same manner. The only way Johnson could've won the 1968 nomination is getting the support of the big party bosses, the same way Humphrey did.
it wasnt even certain that lbj would have been kennedy's running mate in 64.
And Vietnam might have turned out differently had Kennedy been in office until 1969.