There is no informal or official plan as far as I know, and I am quite involved with mountain rescue in the northern half of the state.
Generally in Chittenden, Lamoille, and Washington counties (the area I have responded to), different agencies meet and coordinate on VTAC3. I have been in a handful of operations that were large enough for several VTAC freqs to be in use for different elements of the operation.
Stowe Mountain Rescue and other similar specialized groups use their own VHF simplex frequencies, however I am not sure who holds the licenses.