To address your main points:
HF would not be a bad trip from Montpelier, perhaps 30-45 min drive.
I will warn you that GPS may not get you there accurately and if it does, it may be on a Class IV road which is technically a road (or at least once was).
My friends GPS took him right there with no trouble at all, so perhaps if the software is up to date you will be fine.
Your phone will not work.
Don't trust your GPS. My GPS took me onto something that used to be a road. I drove by it 5 times before I realized where the GPS wanted me to go. It was a 5 ft wide opening in a forest. It was made of dirt. It went straight up the side of a mountain at a 45 degree angle. I don't think a car had been on it in at least 50 years, maybe never. I'm pretty sure there were tracks from a horse and buggy. I'd say it was a Class IX road.
I ended up using my GPS to get to Greensboro Bend, VT. Once I was there my GPS gave me a good route to the brewery. There's a nice little general store there. They had a decent beer selection.
Even the trip to Greensboro Bend was interesting. It took me over a mountain on a dirt road through some questionable-looking peoples' back yards. There were grimy, barefoot kids just standing on the side of the road watching us go by. It was a little creepy. I think I heard Dueling Banjos in the distance.
The brewery was nice though. Worth the trip. The general consensus was to get there Thursday morning. I waited a few hours to get growler fills, but when I left the line was really long.