I've always thought that Vito saw SOMETHING in Carlo that he didn't like, and that was part of his "greatness" (judging people). It wasn't only Tom who was fooled - remember that it was Sonny who introduced Carlo to Connie.

The novel explains Vito's "sight" in some detail... Carlo had been given a "book" (a betting parlor) as his living, but he nearly ran it into the ground through bad management. We're led to believe that Carlo married Connie solely as what he saw as an easy living.

Welcome to the boards, head.