Not Vermont but the White Mountains are pretty spectacular. I'd say early-mid September would still be a tad early for any foliage at higher elevations but still cool. You could go up Mount Washington as well.
For Cape Cod. If you've never been, take the fast ferry over to Nantucket. You can make it a day trip. We did it on our way home from our Washington DC road trip a few years ago.
If you want something better, come over to Nova Scotia and explore the Cape Breton Highlands. You won't get that anywhere on the Eastern seaboard of North America. It's obviously a bit farther but the USD/CAD exchange rate is still around 80 cents so very favourable for you. Heading to Nova Scotia it's even worth exploring taking the ferry from Portland to Yarmouth.