The only “extreme” duty for our car is the cold of Vermont winters. Is it OK to follow the regular maintenance schedule in warmer months and the severe in below-freezing months?
Yup.
I second the same mountain bike’s “Yup”
A couple of yuppies! 
I would use synthetic oil for Vermont winters.
IIRC, Subaru now specifies synthetic oil for their engines.
And, I agree that you can use the “regular” maintenance schedule in warmer weather, and rely just on the “severe service” schedule in the colder months.
I don’t think it is necessary unless it is distinctly defined in the owner’s manual. Generally, if oil change intervals calls are 5k, there need not be any decrease in interval length for Vermont winters. They are little worse then other areas in NE. Some manuals do give you alternate weights for oil under different temperature operating conditions but they also give you a base oil like 0w-30 or 5w-30 that can be used anywhere, and each with the same oil change interval. Would you use severe schedule for states that were warmer ? So, not necessarily unless accompanied by situations like towing, stop and go driving, dusty driving, off roading, heavy idling etc.
Guess I’m the odd man out again, but severe service can apply in the summertime also. A lot depends upon the type of driving, environmental conditions, fuel quality, state of tune, and so on.
OK4450, I agree in principle, but took the OP at his/her word that
"The only “extreme” duty for our car is the cold of Vermont winters".
@ok4450
You aren’t alone. “Severe” cold here in northern NE is a relative term. Winter average temps just below freezing is really quite warm and well within normal maintenance requirements. Dealers who like to have people service cars way too much up here, still don’t recommend severe service intervals in the winter. Now, when mud season rolls around and you have to slug through a foot or so of mud every day getting home for some residents, then taking it in for a little more service seems a little more worthwhile. Otherwise, Vermont winters don’t constitute severe conditions for those who live there ( and here).
We have hot summers and very cold winters, and have use the “mixed” schedule for years with good results. Last time I had an internal engine problem was the summer of 1964 with a flat head Plymouth.