The simplest idea would be to get an ordinary foot vampire and add wings taken from Tyranid Gargoyles or Dark Eldar Scourges - similarly to how
@Mnemloch did with his vampire ladies
HERE and
HERE. Other than that, you could take an already flying model and hive it a vampire face or just a paintjob. One of those flying stormcast eternals with the helmet replaced with a vampire head from Zombie Dragon kit would work... if you could fit it on the base...
I would say that the vampire turns into a bat only to move from a place to place - like vampires in Heroes of Might and Magic 3 - you could explain it to the other player - but maybe that's too simple ;)
Also, this question could find more answers in the Dying to Help section ;) Maybe a mod could move it for you