There is a place that rents VW Campervans in Vermont. If you could get a one-way car rental to get there, you'd have it made.

If you decide to fly out to the west coast and rent one there, there are lots of places in California that rent them. In the Pacific Northwest, too.

I agree that if you're going to start out from the Vermont/New York area, you're going to have limited time to go west. I'd simply explore all there is to see in that area in two weeks. I'm sure you won't get bored. You could also go south.

However, you will have to do a loop drive because I'm sure there's no one-way drop-off options.