What is the relationship between werewolves and vampires in Balkan folklore?

Question

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?

Answer 1

The reason the same term was end up using for two creatures, which were very different, was because they were connected. But not in a sense of family ties.

^{source: pg 800, The Vampire Book: The Encyclopedia of the Undead, J. Gordon Melton}

The reason to be believe so was because, the Slavs and Balkans believed that vampire was a stage that came after the werewolf.

_{source: pg xiv, The Vampire in Lore and Legend, Montague Summers}

Since the vampires were creatures that were werewolves before their death, it is natural for the terms to be connected.

Another myth from the same region:

those who had eaten the flesh of a sheep killed by a wolf might become vampires after death.

_{source: Curtius Wachsmutt, Das alte Griechenland im Neuen, p. 117}

In both cases, the transition to a vampire comes after the event related with the wolf. Hence the base of the term is connected with the werewolf.

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_{Note: As for the transition in appearance and characteristics, I'm still researching on it and can't seem to find the appearance descriptions (no two sources say the same). Other answers are welcome :D}