abirdcall
(she/her)
CHA is the stat associated with persuasion and deception, if 16 INT is smart enough to ward against a patron then 16 CHA is suave enough to smooth-talk them.
Edit: replied to the wrong message, meant to reply to @ad_hoc above
It's about research. The 16 Int isn't doing the warding. I have no trouble believing that a library in a city will have information on folklore.
Van Richten from Ravenloft is usually (if not always?) low level but he is successful because of his research. He knows what the vampires weaknesses are and brings them.
16 Cha enough to deceive an arch fey though or smooth talk them?
If they can do that then they can also succeed on any persuasion or deception checks against nearly all mortal beings.