Thanks Wayne!That's true for the UPC, but Vermont uses the IPC, so unless I missed some additional information, that wouldn't apply to the OP.
IPC just requires the vent to be at least half the size of the drain, and large enough for the DFUs. So if the building drain is 3", it would allow 1-1/2" vents. Of course, Vermont is a cold climate, so from the building thermal envelope up through the roof should be 3".
[I didn't review the rest of this thread.]