Yes, I remember that day.
Always the political observer, even as a kid, I was very impressed by the quick, business-like transfer of power to LBJ, Lyndon Baines Johnson, a man so different from Kennedy in personal style and political experience.
Johnson was a very experienced and wily politician. He got straight to business as President. If he had been the president and Kennedy the VP, suddenly thrust into the role, I highly doubt that Kennedy would have done as well. Don't forget that they had been opponents at the Democratic Convention. This was long before the big primary system got started. Presidents were chosen over deals and there was real suspense during the convention voting.
It was all so long ago...