Vermont's age is 21.
At 18 you can have a rifle or shotgun. No pistol.
That's concealed mind you.
Open carry at 16 with adult supervision.
Whoa, whoa.. you don't need any kind of training or whatever to carry a handgun here. Our age to carry without adult supervision is 16, and that's open AND concealed. It doesn't matter if you have a carry permit in your state, or even in the case of NY a permit to own a handgun. State law here is that if you are a legal US resident, and have the ability to legally own a firearm you can carry. You or your girlfriend can carry any handgun you want, on your person open or concealed, in your car, loaded\unloaded whatever. It's generally regarded as "normal" to OC, but you do run into the odd person who'll give you a stare or try to put their opinion on you, and they're usually not from here. You can not carry into schools, courthouses or federal buildings (yes that includes the post office - mine has a gigantic sign stating the law posted right by the counter). We also have state preemption, so towns like Rutland that tried to restrict where\how you can carry were shot down rather quickly; in other words you won't have to worry about learning each little town's rules too.
As for rifle\shotgun, keep it unloaded in the car. It's a fish and game thing, and you don't want to piss off the wardens here. In fact, our only two firearm laws are fish and game related, the other being no suppressors. Now, unloaded just means don't have rounds in the chamber, it doesn't mean you have to have the magazine empty and stored in a separate compartment. I've kept my AR15 in plain view in the passenger seat with a loaded mag sitting on the seat and gone through a DUI checkpoint - I was asked if it was unloaded, chatted a bit with her about target shooting and went on my way. The police here are a different breed, but always be prepared for the occasional dink.
As for following your home state rules, NEVER follow them in another state, always read up on the laws and rules of each individual state. I'd hate to see you carrying in another state and get jammed up - as they say "ignorance of the law is no excuse."
Anyway, if you don't mind me asking, when\where do you plan to come up here? I live in southern VT, so if you're looking for a place to shoot while you're up here I know a few extending all the way up to central VT.