Lobster shacks we can't miss!
The recommendation I'd make is to take 95 across the bridge to exit 2 in Maine and go to Chauncey Creek. The location can't be beat if it is a nice day, and it is a fun place to go. You can bring in anything they don't serve (alcohol, salads, desserts, etc. - they are pretty liberal with this policy). You go down the ramp from the street, pick a table, and then go to order. You'll need to give them your table number when you order so they know where to bring the food. It's not a huge distance from 95, or I'm sure you can find some nice back roads to take over to VT.
I actually prefer the food at Markey's (see my comment below), but as much as I love food, the location usually wins out in this case. Here's their web site
dfrost has some great recs as you leave the coast. We really love Lobster and seafood in Seabrook at Browns (it's a byob so bring your favorite bottle or six pack) and Markey's is just across the street and have heard great things as well (might not have steamers if you want the whole clam bake dinner) but others should advise. You'll have a blast, it's the nice thing about new england, you can hit 3 or 4 states in a couple days and really enjoy all that it has to offer. We just did a kayak tour out of Newburyport (plum island), was fabulous if you get into that area too. Lots of great rest.
Markey's will definitely have steamers. People in this area are either Markey's people or Brown's people. I am a Markey's person. I think the lobsters there are great and their fried seafood is good too. They are both very casual, but Markey's is a tiny step up from Brown's. They do serve beer and wine at Markey's.
We haven't had steamed lobster in the Portsmouth area recently so I'll let other chowhounders make recommendations for a lunch stop. I still like Newicks on Dover Pt Rd since it sits on the water and has been there in one shape or another for many years. It's big but a paper plate kind of place with quick service and an extensive menu heavy on fried seafood. From Portsmouth you would head toward Concord on the Spaulding Turnpike which takes you over the Great Bay bridges. This is the route I would take to VT for the following reasons. From Spaulding Turnpike take exit for Rt 4. This will take you by Johnson's Dairy Bar in Northwood where it is customary to stop for ice cream. Do not order a large cone! We like the seafood here too but the cole slaw is terrible. It's a pleasant ride to Concord. As you near Concord, do not get on the Rt 393 bypass. Continue into Concord on Loudon Rd. You're going to go by all the chains but keep your eyes peeled for Arnie's. It will be on your right and looks like a former Dairy Queen. They make their own ice cream. Scoops are on the small side so it's ok to order at least two scoops so you can try two different flavors. Bananarama is one of my favorites: banana ice cream with peanut butter chips. Arnie's also does their own barbecue. Continue into downtown Concord making a left hand turn onto Main Street (Holiday Inn is on the corner). You might want to shop downtown Concord i.e. Chutters for wine and cheese. If you keep walking or driving, Bread and Chocolate is on your left next to Gibson's bookstore and the brand new office building. They have European style pastries and wonderful pre-made sandwiches on their own bread. Maybe you need to buy some muffins for tomorrow's breakfast? But I highly recommend their sweet bread with nuts and chocolate or poppy seeds. Across the street is the Concord Food Coop which has grown and remodeled into a lovely small grocery store selling organic food. This is a great place to buy herbs and spices since they are sold in bulk. Just measure out what you want into small plastic bags. This is where I buy whole cloves for a family friend who makes iced spiced mint tea in gigantic portions. About a half cup's worth was half the price of what she was paying in the regular grocery store. Before you start your trip, check the menu for In a Pinch Cafe to see what they are serving for soup specials. The cold peach is heavenly. All soups are very good. After leaving Bread and Chocolate you can either turn around by driving thru the Food Coop parking lot or take your next right then right again on State Street so you can take a left onto Pleasant Street. (Follow signs to the hospital) In A Pinch is in a white house on your right across from the state hospital grounds just before the intersection of Fruit Street. They close at 3pm - just open for breakfast and lunch. Small and very busy place. Continue west on Pleasant St which is Rt 103. It will take you into the town of Hopkinton. Look for signs to Gould Hill Orchard. It will be a right hand turn. Marvelous view from their place and they sell old apple varieties and peaches. There's a little nature museum in their barn. Return to Rt 103 which sort of parallels Rt 89. Just watch for signs to Rt 89. This will take you to VT. Stop at King Arthur Flour in Norwich VT for unusual baking supplies.
Of course, it you are headed to southern VT, ignore these directions.