It's important to note that a key part of the werecurse's virality is that it transforms the victim into a generated werebeast. Generated werebeasts have [ATTACK] tags which have [SPECIALATTACK_INTERACTION] tags which add the werecurse bite interaction to the victim. You could do something similar with a vampire, but you'd have to create a vampire creature for it to work. Your best bet, going down that route, would be to create a creature variation template which adds a "sire" attack, which would expedite making vampire variations of humanoids.
That said, there might be an easier way. I'd give vampires a [CE_CAN_DO_INTERACTION] token in the curse syndrome which links to the vampire interaction (you'll have to define the interaction as also a [I_SOURCE:CREATURE_ACTION], and be clever about your targets), and then give the interaction a short range. Make it touchable targets only, attack usage hint, and give it a cool adventure name (like sire vampire). Finally, I'd try adding a delay to the start of all the syndrome effects (of a few days to about a week) so that only those who survive an initial encounter with the vampire will become sired.
If you wanted to make the curse still activate instantly, you could split the siring interaction off into a new interaction, copy paste the effects from the curse (including the [CE_CAN_DO_INTERACTION] token I mentioned above), and add the delay to THAT. I am not going to guarantee that this would work. One challenge you might face with this method is making it so that the victims don't get renamed until after the delay is finished, but there might be a workaround for that too.
Good luck!