Difference between Vanilla and Pucci was that Vanilla acted like a dog for DIO. He never thought about doing something outside of his orders, that's the reason why DIO put Pucci over everyone else. Pucci understood the idea of heaven and that was the reason behind his actiond and why he was so loyal to DIO's intentions, unlike Vanilla he can act on his own. Pucci never fell in love with DIO and same goes for DIO.
