Reviving this old baby as I hate starting new threads. Wanted to throw it out there that I effectively live on this island now (when I don't spend weeks on end on the road for work). I've gotten to know the off-road options well and have been having fun this summer.
Always happy to give tours to folks, I accept high fives and beer as payment. I live in Vineyard Haven so its easy to meet folks at the boat and there is plenty from there.
Alternatively I also used to live in Falmouth and know the area well, always looking for excuses to grab a boat to Woods Hole and mess around on the cape.
As a heads up if anyone ever did want to ride on the island come prepared. While there is a lot of dirt (with lots of sand content) most stuff is doubletrack and singletrack can be boring in stretches. A lot of the riding is more about exploring and pedaling than playing or descending, though it happens. Most stuff requires many bike path, road, or dirt road connections.
I use my plus HT and its a blast. As someone who travels the east coast to ride and loves rugged big days and elevation and all that; I'm always pleasantly surprised here. There is climbing (50-75ft/mi avg. vs more typical 100-200ft/mi on mainland), rocks, roots, places to boost, technical sections, flowier sections, etc. plus ocean views.