It has all been the fans. Those who love the characters have been the ones to make them sexy. Without the fans, Barnabas in Dark Shadows would have ended up with a stake through his chest. There would have been no Louis in Interview With a Vampire (sorry, Brad Pitt) and Angel wouldn’t have made an appearance.
Before the 1960s, vampires were the predators. It made sense for them to be sophisticated and make people want them. It brought a sense of trust and desire so the vampire could attack discretely, quickly and easily. However, the fans turned them into love interests. They have led to the use of them in love stories.
However, none of this takes away the truth that vampires are undead men searching for blood. They are parasites without a heartbeat. When you consider that, how could they ever be sexy? But somehow the fans look past all that and just see the men on their TV screens or in their minds to make vampires sexy.
By the way, the same is working with werewolves. People now view them as sexy men by day and wolves on the full moon. Without the fans, they would have remained cursed creatures.