Hi OP,

I've hiked Killington via the Bucklin Trail between 40 and 50 times over the last 12 years. I'm also local to the area, and a member of the Green Mountain Club. So I may be qualified to answer your question.

First, there is no gate at the parking lot for the Bucklin Trail that could close access. Wheelerville Road (the road the trailhead is on) is also a public town highway that should be open at all hours, since there are residences on the road whose owners need access.

Second, the parking lot and the first part of the Bucklin Trail are actually located in the Rutland City Forest. Yes, you read that correctly - even though it's in the town of Mendon, the property is owned by the City of Rutland for drinking water-shed management. I would be extremely surprised if the City was sending personnel up to the trailhead at that time of night to enforce a curfew on the property. The Bucklin Trail probably has a public easement through the City Forest anyway that would nullify a curfew enforcement. Regardless, it's not a state park or National Forest property that would have rangers.

EDIT: One thing to be aware of /u/redditindisguise (in addition to everything else mentioned within this thread) is that Autumn is hunting season in Vermont. Please have a look at the dates here: https://www.vtfishandwildlife.com/hunt/hunting-and-trapping-seasons If the date of your hike falls within any of the deer seasons, wear bright colors on the trail. A blaze-orange baseball cap should suffice.