from CT building code:

"... 917.7 Vent required. Within each plumbing system, a minimum of one stack vent or vent stack shall extend outdoors to the open air. ..."

Do I read this correctly that when the AAV vented drains drain into my secondary stack, this stack (or at least one waste line connected to this stack) should be vented outdoors?

You don't have a plumber. You have a cheap hack. NO, it's not OK to vent a main stack with an AAV. Never has been anywhere, so it's not like this is an unsupervised apprentice going "oops, it's OK is Yokeltown so I assumed it's OK here." type of situation. A licensed plumber would know differently. A licensed GC would know differently. BIG RED FLAGS!

thanks- I am learning it the hard way.

The stack in question does not require any vent..This stack is serving only the kitchen and laundry, and they will both be fitted with AAV's, therefore the stack is now only a vertical portion of a vented branch and it does not require a vent.

the laundry sink does not have any vent at the moment.

The two kitchen sinks will have AAV vents.

the two kitchen sinks are on the first floor; the stack and laundry sink are in the basement. Is this still "vertical" portion of the vented branch?

Yes, and if the utility sink has an 1-1/2" drain and it is not more then 6ft from the trap to the stack it does not require additional venting.

AAV's are one-way valves, and therefore do not handle positive air pressure. This is why it is best practice not to rely on AAV's, in general.

A strong argument can be made that your kitchen + laundry stack (since it's vertical) might drain poorly at times depending on the activity in the vented stack, from tubs toilets and showers. More than that I cannot say.