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.