I think we probably do need a basic principle enshrined in law that prohibits exploiting the vulnerability of someone who, for whatever reason, is not capable of making reasonable decisions or of acting to protect themselves in a normal way.

What follows from that principle, and where any lines or grey areas are drawn, obviously depends on the specifics of the situation. I'm not generally in favour of having lots of overly specific laws. I think in most cases, responsible adults will agree on whether any particular action has clearly crossed the intended ethical and legal line anyway. Where there is more room for reasonable disagreement, that's what courts are for.