@ally8173 hiii, you could say that it's one of the sacrifices needed to sustain the dark magic that lives in vampires(like magic always has need for sacrifices and always has side-effects:vampirism was created as a protection against werewolves, werewolves are normal during the day and bound by the moon, vampires are 'normal' during the night and bound by the sun. since it also has to with protection against sicknesses, injuries, etc. and the thing that reoccurs everytime in them, consuming blood is also needed, and the og's had to consume tatia's blood to start the transition and drink someone else's blood to finish it, and in mythology, magic, etc blood is also called life essence, and many more things. , and i'd say a blood bag a day or every two days would sustain them since in tvd they always asked for one blood bag or smth. SORRY FOR THE LONG RANT, HOPE THIS HELPS