Vampires have always been popular romantic figures, even when depicted as monsters. The roles have always been played by handsome men and beautiful women, and a major element in their characters has always been their sensual attraction. Even the idea of heroic vampires was established well before Angel or Spike ever came along, in books like "Interview With A Vampire," "Old Friend Of The Family," and "The Dracula Tapes," in TV series like "Dark Shadows," and "Forever Knight," and in movies like "Dracula: Dead And Loving It," "Love At First Bite ," "My Best Friend Is A Vampire" and "Innocent Blood," all of which were around long before Buffy' series premiered.
Certainly, Buffy contributed to the new trend of vampire popularity, but I seriously doubt that that contribution was of such an importance that, without Joss and his characters, it wouldn't have been just as popular.