since getting the vampire disease or werewolf disease can be such a long wait and people ask money for them and most of the bite offers are from scammer
because most people are willing to pay for the bite why not put a potion in the crown store that gives the disease once they drink it they follow the quest just like someone who got bit no changes just a different way to get the disease
i'm not sure how the lore is in eso but in several vampire stories people who drink the blood of a vampire become one themselves so in my oppinion it shouldn't be a problem lore wise
and for pricing i would say the same price as a skill reset scroll i guess or other potions in the store not too expensive but also not to cheap
anyway i think it would sell very well and it would help alot of people play the game the way they want to be it for min/maxing or role playing