ABOUT LUFFY'S VULCANIZATION:
If you research vulcanization you will learn that it makes rubber way more resistant to heat and cold! This means that if Luffy vulcanized the right way, he might be able to beat Aokiji and Ace!! Oda is the kind of guy who always researches what he's doing, so you better believe he's looking into every possible way to improve Luffy. If you want proof of this, look at how Enel actually uses his electric powers to solder metal and uses gold as a conductor, or how Smoker uses his smoke powers to ride the "Smog Hog", or how Aokiji rides the bicycle on the ocean, or how Mr. 3 uses it to create sculptures...... he's always thinking of how to apply the Devil Fruits, and rubber will be no exception I'm sure.
ABOUT DEVIL'S FRUITS IN GENERAL:
I think that Oda will explain in the future that anyone who eats a Devil's Fruit was actually chosen by the fruit, and that it was all fate. The right fruits go to the right people and give them the right powers. There are no coincidences or accidents. And I think I read in a translation of an SBS (on the Arlong Park website) that if someone with a Devil's Fruit even gets close to a different Devil's Fruit he'll feel so sick that he can't eat it -- so tricking someone into eating it would be impossible.
Anyway, I really wonder why Chopper never offers a rumble ball to Luffy or Robin. It should work on more people than just Zoan types, since it just shifts wavelengths. Hmm.