Smart guy you are
10% of arguments are caused by disagreements, while the other 90% of arguments are caused by the tone of someone's voice..

It is ALL about approach.
The guy probably isn't trying to rebel....he probably just doesn't think vaping does, or should bother anyone. You see that very attitude on this board all the time.....people upset because someone asked them not to vape in their store or whatever.
Many people feel entitled to be able to vape anywhere, I guess because they figure it is vapor, not smoke (you and I both know otherwise, but some people don't).
I would politely spark up a discussion with him, and very nicely ask him if he wouldn't mind waiting until he went outside to vape, since the vapor might irritate and/or bother some other customers. Then you can let him know you vape as well, and have a bit of convo with him. More than likely, if you are polite about it, he will be cool.