Perhaps better to say Dio is not very practical; you're right, he's at least intelligent enough to graduate law school. If he was, he could have easily hijacked the body of...anybody, rather than Jonathan and had his revenge at his leisure (or not at all, and bided his time until Jonathan died of old age).
Or ****...never become a vampire in the first place for that matter. He had absolutely no reason to kill George.
The man's pride has ALWAYS been his downfall.
And no, it wouldn't (or shouldn't) get ignored by DB. They've always said one of the "rules" (at least an informal one) is all characters retain personality traits save the removal of any unwillingness to kill.
He never encounters anyone with the ability, so who knows. Even if he had it wouldn't really denote anything, since Stand abilities tend to be a lot more ABSOLUTE than most fictional superpowers. A Stand with the power to hypnotize people in most other settings would theoretically bypass any resistance to such.
Dio may or may not have hypnotic powers of his own, but if he does (and it's not just that he's DISTRACTINGLY SEXY which also seems to be implied) they're pretty weak, as eg. Avdol manages to fairly easily break out of whatever slim hold he may have over the human psyche.