The character of Professor Vambery is loosely based on Armin Vambery, alias Hermann Wamberger, a Hungarian orientalist and geographer, who met Bram Stoker and somewhat inspired the creation of Dracula.
References to Lilith are often found in vampire stories, as is the suicide of Dracula's wife, a subject dealt with by Coppola in his version of Dracula.
The captivity of Vlad and Radu is a known historic fact. Radu III, sometimes referred to as "Radu the Handsome" or "Radu the Fair" was indeed one of the minions of Sultan Murad II, and was released 14 years after his brother. It was upon release that he went ahead and fought a campaign against Vlad to take the throne of Wallachia.
The storyline of course makes reference to the "Portrait of Dorian Gray". Although Oscar Wilde's masterpiece was penned prior to Stoker's novel, the two men knew each other, having studied together at Trinity College in Dublin.