Marvin Pancoast, serving a 26-years-to-life term for the baseball-bat slaying of model Vicki Morgan, is being treated for AIDS, his attorney, Arthur Barens, said. Barens said he believes that Pancoast, 35, an acknowledged homosexual, contracted acquired immune deficiency syndrome "well before" he was sentenced to prison last year for the July, 1984, first-degree murder of Morgan--mistress to the late Alfred Bloomingdale, a department store heir and friend of President Reagan. Barens said Pancoast has been placed in a special wing for AIDS victims at the state prison at Vacaville. "He's philosophical at this point," Barens said. "His philosophy is to take it a day at a time." But another attorney, Charles Mathews, said Pancoast expects to die and wants him to stop the appeal of his conviction.