Have you checked out hipcamp.com, kind of like an airbnb, but for rustic cabins and might be able to book for longer period of time. Worth a shot.

Maybe Hubbard Park could work - there are several pavilions, walking trails, feilds...etc. https://www.montpelier-vt.org/238/Hubbard-Park

Bar Hill Distillery is an awesome location on the river and urban / chic setting with patio / private function space. Definitely check them out.

https://caledoniaspirits.com/visit-us/#privateevents

Maybe call Morse Farm and Sugarworks as well, they might have barn space / function space.

Zen Barn in Waterbury does small ceremonies too.