All this is to say I think it's highly likely that the assassination is butterflied away. And even if it's not, maybe JFK appoints someone else as VP first, so whoever that is becomes President instead of McCormack. True, LBJ didn't bother to appoint a VP until he and Humphrey won the election, but maybe JFK would if it happened nearly 2 years before the next election instead of less than 1.
There was literally no way to appoint a Vice President before the 25th Amendment... it's not that LBJ "didn't bother".
