Could you please post the source - I'd love to see who came up with this one.
Unlees I misunderstand what you are saying...
Yes there are mites and fleas which prefer to parasitize just a single species of bats.
But the vampire bat's feeding habits have NOTHING to do with a virus infection changing what/how they eat.
Ever been bitten by one? I have. There's a lovely 45 year old scar on my forearm to prove it. The bite is painless, and continues to bleed long after Mr Bat takes off.
The bat has evolved over long periods of time to be able to open a wound and lap up blood successfully. A big part of that feeding strategy is not getting killed while feeding.
And it is possible for a bat to transmit a virus during feeding. Whether the virus can reproduce in the new host is extremely problematic.