The writer met Howard in 1950 and they remained close until he passed away last year (12).
Their long partnership baffled Hollywood, but during an interview conducted before his death, Vidal told Vanity Fair magazine the secret to their success was staying out of the sack.
He said, "All the f**got fan magazines wanted to know how these two famous f**s stayed together all those years. They wanted to know what was the basis of your relationship. So I told them. We never had sex. I had sex and he had sex, but it was never together.
"Of course, they were furious and hated me for that answer. Howard was living with (writer) Rona Jaffe when we met, and was with her for almost 10 years until he moved in with me. We never had sex, because I'm more intelligent than most people, and that's the way I wanted it. I was smart enough to know that if you wanted to keep a friendship you made sure there was no sex. It was self-preservation."