Leaf-peeping season anywhere in New England is a very busy time, and Vermont is no exception. You are ending up north of I-89, which will help some - tourists from New York and Connecticut especially, but Massachusetts too, tend to visit the southern part of the state. But no reservations is still a risky approach.
You can get foliage reports, updated daily, on
http://www.vermont.com/foliage.cfm. This will tell you where the most spectacular colors are at any given time, and therefore where most of the people are.
To the extent that it is possible, you might consider making unpaid reservations as a backup. Even if you have to pay in advance - and you probably will - bear in mind that Vermont is not a very big place. If you know even approximately where you want to be, you won't be outrageously far from your reserved campground even if your plans change a bit.
Bill