That's alright, just a different sort of Dr is all, like I don't think a scientist is a "Doctor", they just have a "Dr" title for having just a doctorate, because the "Dr" just means that they can teach their field at a degree level. For example you can be a "Dr" by title in archaeology or history, so I'm guessing Vegapunk just falls under this category and not the medical terms.
EDIT:
I will say point made, but that still doesn't conclusively say anything in relations to his doctors, however, since you like patterns so much, Ceasar is a scientist and gets called by his last name? Soooo by your logic wouldn't Oda call Vegapunk by his last name too?