There's no need to deactivate them in an attempt to same VAS when your not flying to one of them. Do this as a test, activate all of them and start a flight somewhere and measure your VAS using a freeware tool. Then quit the flight, deactivate all the FSDT airports and quit FSX. Restart FSX and go to the the same airport you used in the first test, using same time, date and weather and measure your VAS again and it will be the same.
FSDT airports only load when you get around 10 miles or so from them and once they load they also unload we you exit the radius. If your concerned about disabling things to save VAS which is the big thing now, the main thing do disable is photoscenery since it will load into memory no matter where you are. I own a bunch of photoreal scenery for SoCal, Vegas, Phoenix, Chicago, New York, etc. If I keep them all load and go to top down view and zoom out to view the whole US, I can see all the photoreal areas load across the country and that consumes a lot of VAS.