“A River Runs Through It” star Brad Pitt has committed in principle to TriStar Pictures’ $ 35 million production “Legends of the Fall,” a deal that hinges on whether David Geffen Co. and Warner Bros. agree to push back the start on “Interview With a Vampire” to as late as Oct. 1.While WB officials declined comment, a studio insider confirmed that production of “Vampire,” to be directed by Neil Jordan, will not begin as planned in April and won’t roll until August or September. By press time yesterday, TriStar was still awaiting official word from Geffen on the status of “Vampire.” Geffen declined comment yesterday through his publicist. TriStar submitted an offer for Pitt to star in “Legends” on Tuesday night, which the star’s Creative Artist Agency and Brillstein-Grey Entertainment management team informally accepted — contingent on the start date for “Vampire ,” studio sources confirmed. Pitt’s camp declined comment yesterday. Based on Jim Harrison’s novella, and to be directed by Ed Zwick, “Legends” spans the lifetime of a tough English colonel and his three sons who leave their ranch in Montana to do battle in World War I. When the youngest boy is killed, the family becomes severely fractured. “Legends” sports a below-the-line budget of roughly $ 20 million and is earmarked to begin production by June 10. Principal photography was originally to begin on “Vampire” in April so as to avoid a potential conflict over the rights reverting back to author Anne Rice, who wrote “Interview,” in early May. Reached yesterday at her home in New Orleans, Rice said she had “come to terms (with Geffen) and extended the rights deal.” Because the negotiations are “confidential,” Rice said she could not divulge the terms of the extension, which is believed to be for an additional year. The postponement of “Interview” could presumably buy Geffen more time in getting a commitment from Daniel Day-Lewis, whom the producer wants for the role of Lestat. Day-Lewis’ agent, William Morris’ Gene Parseghian, told Daily Variety last month that his client was not attached to the project, as had been continually reported, and that the actor would not even read potential movie material until May, after he completes Jim Sheridan’s pic “The Gerry Conlon Story” (Daily Variety, Feb. 16). While Pitt was planning on “Vampire” being his next movie, TriStar intercepted his attention when writer Susan Shilliday delivered a rewrite of “Legend.” The studio immediately put the project on the fast track, following eight years of development. The project was originally adapted by “Lonesome Dove” screenwriter Bill Wittliff under the aegis of former executive VP of worldwide production Stephen Randall. “Legends” has since been taken up by studio president Marc Platt and president of worldwide production Stacey Lassally.