Hey Matt. That ferry is out of Grand Isle, so head up Rt 2 from the Burlington area and follow it out into the Champlain Islands, then follow the signs to the ferry. Here:
http://goo.gl/maps/FOJQIAlternatively, if you have the time you could ride Rt 2 all the way through the Champlain Islands and cross into New York by bridge just below the Canadian border. It's a nice ride, it's free, and it would probably still take less time than taking the Burlington-Port Kent ferry. And only adds about an hour getting to Plattsburgh versus the Grand Isle ferry. If you do that, make a left off Rt 2 at the bottom of the bridge onto Rt 11, then keep going straight through the town of Rouses Point, which will put you on Rt 9B. Follow that till it hits Rt 9 and make a left. Rt 9 will bring you right through Plattsburgh.
Not sure where you're ultimately planning to go, but if you want to take a nice scenic ride through the Adirondacks, turn right onto Rt 9N when you get to Keeseville, just past Ausable Chasm. Follow 9N all the way to Keene, where it merges with Rt 73 (if you're looking for a great spot to stop for lunch, the ADK Cafe right by the intersection of 9N and 73 in Keene has great food, including the most amazing Reuben sandwich I've ever eaten).
Or if you have more time and want a longer ride, you can turn off Rt 9N in Jay, onto Rt 86, and follow 86 down to Lake Placid, then turn left onto 73 and follow that into Keene. There are some nice twisties and awesome scenery along 86 and 73.
Either way, then follow 73 south, over the mountains, to where it merges with Rt 9. If you want to head east again, you can make the first left after you pass the I-87 interchange (you'll recognize that particular twisty road) and work your way over to the Crown Point bridge to get back into Vermont.
--mark