NOTE to the Grim Reaper from Dominick Dunne: Scram! “It ain’t my time yet,” the Vanity Fair writer, who’s fighting bladder cancer, told Page Six. “I’m feeling much better and everyone tells me how great I look. I’m still getting treatment, but I’m doing really well.” Dunne, 83, who’s finishing his new novel, “A Solo Act,” says he’s been heartened by fans’ get-wells on dominicksdiary.com, including one from “Marley & Me” author John Grogan, who wrote: “You provided me one of the big thrills of my early years as a reporter. I was covering the William Kennedy Smith rape trial in Palm Beach for the South Florida Sun-Sentinel . . . You were already long a literary legend in my book, and I was thrilled to be able to exchange a few words with you outside the courtroom. I hung on every word you said – and wrote . . . Thanks for making a young journalist’s day! I’ll never forget your pleasant camaraderie.”