Stumbled across this, and didn't see it already in the forum
Signage example photo and thread: https://www.reddit.com/r/mildlyinteresting/comments/pjn0hz/vermont_has_a_highway_system_for_atvs_and/
Trail map, with route number layer: https://vtvast.org/trails.html
The route numbers are based on nearby road numbers, but are "off in the woods". There are many suffixed and branch routes. Clicking on a route marker brings up more information, including the club that maintains the route.
Corridor 9 is in the general area of VT 9 (to the south). Other numbers like 4, 7, 100, 142, etc. call out to nearby highway routes.
It's another set of routes to clinch!
Yes, the route numbers very loosely correspond to adjacent/parallel state routes, but there are some exceptions (trail route 101 in Lamoille County, for example).
Also worth noting that the VAST map is just for snowmobile routes. In my experience, the ATV routes have a completely different numbering system as well as not being as robust as the snowmobile routes...many towns do not allow ATV use.