In an unmodded game they'll only drink when their thirst reaches 0 and they get the "irresistible thirst" moodlet (or whatever it was called). Then they should lose control and drink heavily from whichever sim who's available. They'll also drink autonomously if they are in a club which has "drink plasma" as club activity.
I wish we could more easily actually see vampires acting like vampires without us needing to specifically direct them. They never even use their powers (except movement ones). Even the nighttime break-ins got nerfed to basically non-existency so that I had to use a mod to restore them. *sigh*
