In Balkan folklore, several words that originally meant werewolf (e.g. the Bulgarian vǎrkolak and the Serbian vulkodlak) either ended up being used for both creatures or for vampires alone (e.g the Greek word vrykolakas, a derivative of vǎrkolak). This - to me - suggests that the concept of the vampire is strongly related to and possibly derives from the concept of the werewolf.
Is that true? And if so, how did the transition from werewolves to vampires happen? Was it straightforward or was there an intermediate creature that combined characteristics from both species?