Isn't the whole reason Neelix called Tuvok "Mr. Vulcan" because the introductions were something like (paraphrasing):
Neelix: "And you are...?"
Which Neelix took to be Tuvok's last name, as in "Mr. Tuvok Vulcan"?
If that's true, I can't blame Neelix for getting it wrong. Of course, he never corrected it after hearing NO ONE else call Tuvok that, when he did hear people say "Mr. Tuvok."
Still, I still call a nephew "Baby Sam" even though he's at least 25yo and near 6' tall. But when I met him, he was only 3 years old and quite short. The "Baby Sam" name has continued in much of Hubby's family, too, since there is (and was) an adult Sam.