There's not really a quick way to cut across. If it were me, and I used to drive from Portsmouth to Burlington fairly frequently, I'd take either 101 over to 93 and onto 89, or use Spaudling/202/4/393 to get to 89, then take 89 up to the ferry north of Burlington (The name escapes me, but it's off of 2 in either Grand Isle or Hero...), which runs all year over towards Plattsburgh, and then head from there over to Lake Flaccid via any of several routes (3 or 9N/86).

If you want to avoid the ferry, get back onto 4 in WRJ, VT and keep on it until Whitehall, NY, and then cut north into Ticonderoga and over to LP.

Personally, I'd take the ferry, as it's fast, cheap, and runs fairly regularly. No matter how you do it, it'll be a looooooooong drive.