Vancouver could be a first stop from a starting point anywhere in the lower 48 states or the penultimate leg.
The concept of driving from Boston to anywhere in the Maritime Provinces is a bit of a stretch for me. Do you know how much driving that involves? It takes 2 hours from Boston to Portland Maine. From there your options are 4 days a week (including the ones most likely to be a starting day for an Amazing Race) to take the ferry from Portland to Yarmouth, Nova Scotia for 5.5 hours and then drive a another 4 hours up the coast to Halifax; to drive 6 hours to that huge (NOT) provincial capital of Fredericton, New Brunswick; or to drive 6 hours to St. John, New Brunswick. It's not impossible but it is quite improbable.
From Boston, it is much more likely that teams would fly to Montreal, Quebec City, or Halifax.