I don't know, it doesn't convince me, not for your ideas themselves about her abilities, but for the lore accuracy of the whole concept.
The
Vampires, that Tolkien refers to in his texts, are mysterious and evil bat-like creatures that have probably the ability of shape shifting in both 'humans' and giant bats, and that were at the service of Gorthaur/Sauron, who could by the way turn in a vampire as well.
The problem is that the Vampires were only present in the World in Beleriand, during the First Age, legendary times for the people of the Third Age, in which immensely terrible powers used to clash both for the Good and the Evil; it is likely that after the War of Wrath all these creatures (Vampires, Werewolves,...) were totally slain by the Host of the Valar, and thus became elements of the legends of Men of the Third Age, as it often happens with characters or events concerning the Past of Middle Earth).
Furthermore, the particular power of shape shifting is deeply related to the Valar and the Maiar, a power that Gorthaur/Sauron already has in the game.
I personally would prefer more a sorceress concept, or something more related to the universe of Men in the Third Age
Umfrage