Two thoughts for you.

1. Contact Underhill State Park to see if staying there is an option 802.899.3022 - their season runs through 10/15 but after-season accomodations can still be made with the ranger if made in advance.
2. Visit this website and scroll to bottom for PDF maps specific to Primitive Camping in Mount Mansfield: https://vtstateparks.com/camping.html#remoteCamping