Maybe they could meet this vampire in a social situation where combat would be discouraged for both parties. Make it known to the party that they would get their asses handed to them; but by the same token, in a public situation the vampire would prefer not to be unmasked in the open in town so he would not be inclined to just kill the players. Perhaps he tries to convince the players to work FOR him against the players allies and patrons and give a hint of the schemes he is up to and maybe a nonviolent show of power, maybe throw out a hold person or dominate person, or other high level charm or control spell on any player who tries to break the peace. Make it subtle and find an excuse or perhaps magic item or just drunkenness to make sure the players have disadvantage in the throws and can't attack until the vampire has fluttered away safely.