Hoffman's Accused Dealer -- I Didn't Sell Him Deadly Heroin

The guy accused of selling Philip Seymour Hoffman the heroin that killed him SWEARS he didn't do it -- but he wouldn't deny selling heroin to Hoffman in the past.

Robert Vineberg -- who did an interview with the NY Post from inside jail over the weekend -- says he hadn't seen Hoffman since last year ... long before the actor passed away.

If he had known Hoffman was in town the weekend he died, Vineberg says he would have tried to "save him" because he knew the actor had a serious heroin problem.

Which sounds like BS ... because when Vineberg was asked if he had ever dealt heroin to Hoffman in the months before the actor's death, Vineberg declined to comment. Nevertheless, Vineberg insists the heroin Hoffman injected immediately before his death was NOT from him.

As for Hoffman's drug use, Vineberg seemed to know a lot about it, claiming he's convinced Hoffman was using 10 bags of heroin a day.