Looking at the vampire transformation, one can look at it like a virus that changes the body’s ability to tolerate sun, the need for blood, the ability to fight off disease, fast healing, and longevity.
The thought is that the viral particles of the vampire virus consists of a viral jacket that is wrapped around a strand of DNA or RNA. The jacket has feelers that bond to the outside of a cell, and once it lands on the mortal cell, it can inject its own DNA into the cell, where RNA polymerase interprets the viral commands and ribosomes create the enzyme that it wants.
These can create new protective jackets around the replicated DNA and burst from the cell to invade surrounding cells.
The vampire virus would be able to disable some of the normal mortal genetic processes.
There has to be some sort of genetic change to change a mortal body and make it susceptible to damage and disease, which is why a viral transformation is plausible if the mortal’s blood and body comes into contact with vampire blood.
The vampire blood would have to change the DNA sequence by either a base substitution (inserting one subunit of DNA strand where another should be), base addition (adding one more than needed), or base deletion (removing one that should be there).
These changes can cause what a person becomes. The vampire transformation must involve some sort of genetic mutation or chromosomal rearrangement to change the traits of the mortal into a vampire.