Did you look at travel times vs distance? Even though it's only ~30km, Google Maps says that going though South Burlington on the Montreal<->Albany route adds almost an hour and a half.
No matter how you do it (going under Lake George, or the bridge at Crown Point), it looks like you're spending a lot of time on VT back roads. The ferries would get you back to 87 more quickly, but I have no idea the cost or time involved.
Seems to me that South Burlington is a poor substitute for Plattsburgh (are you listening, Tesla? :wink
EDIT: Tabarnouche beat me. Yeah, what he said.
EDIT #2: See
Lake Champlain Ferries | New York Vermont Ferry Crossings | Lake Champlain Transportation
Looks like the Charlotte-Essex ferry is open year round (?), $10 car & driver, 25 minutes to cross. Might save you 25 miles of back roads and get on 87 20 miles further north? The ferry out of Burlington has a 1.5 hour crossing, so that might not save any time.