I can only speak to the Vermont options. As others point out the mountains are why East-West routes without too much climbing are limited. Because of that, In Vermont, at least, they’re relatively busy with lots of truck traffic. Route 2, especially, is a bear in places - with little to no shoulder.
Check out the Cross Vermont Trail. It utilizes quieter roads, rail trails, some gravel and, yes, parts of Route 2, without a lot of climbing:
Cross Vermont Trail Association - Home Page
Route 9 in Vermont is pretty busy (by VT standards). I haven’t ridden it but my impression from driving it is that it’s better than Route 2.