'LBJ Assassinated'.
More specifically, some time after JFK is offed, rather than on the same day.
If Johnson dies, Hubert Humphrey would become president. Civil Rights under Humphrey would be interesting, but I don't know enough to give a good opinion.
For speaking about Vietnam, I think that it would depend on when Johnson dies, I think, pre or post Gulf of Tonkin. If Pre-Tonkin, I don't think Humphrey would have escalated Vietnam to the point at which Johnson did, from what I've read, his support for Johnson's escalation was more out of party loyalty and support for the President's agenda than his own convictions. If post-Tonkin, iirc, Humphrey if elected, did plan to make a speech promising to end the bombing of North Vietnam.
For more social-cultural issues with LBJ being assassinated, it could depend on who kills him. Maybe it's a racist mad over the civil rights act, maybe it's an antiwar protestor.
I also think it could also trigger a re-evaluation of the Secret Service's protective mission. After all, they've lost two presidents in under ten years.