I've got so many Beatle books and magazines, it's virtually impossible to put my hands on the direct source anymore.
From memory, John and Paul had been seeing each other frequently around that time, and Paul invited John to play on V+M. John accepted.
Paul also saw Yoko around that time. I don't remember if Yoko contacted Paul, or Paul contacted Yoko, or if they just ran into each other. Somehow this meeting between Paul and Yoko led to Paul helping to get J+Y back together.
John went home to Yoko, and he never got to play on V+M. He sent his regrets by postcard... somehow a postcard was involved.
This is why I put forward the hypothesis V+M is about J+Y. Like I said, I bet Yoko told Paul "Venus and Mars were in retrograde" or something, which "made the time right" for John to come home. Yoko was heavily into the stars, and Fred Seaman talked about how she would make decisions based on which planet was "retrograde".
The meaning of V+M (or at least what inspired it) is all conjecture on my part, but putting the song the people and the events together seems to make some sense.
