If you're already flying part of the way, ie. from DCA up north, I think you're crazy not to fly all the way into the Vineyard vs. dealing with all the hassles of getting there by car.

The idea of renting a car, driving to the ferry (getting a ferry reservation for yuor car is a challenge to being with), paying for your car to hop over, etc. is a bit much if you can avoid it.

A couple of years ago, I took a non-stop flight on a from DC to the Vineyard. I think it was on a NW prop (might have been CO or US)...I sat next to some FL Senator's wife going up to join the Kerry's for the w/e in Nantucket (the plane continued on to Nantucket).