While my closing process is not perfect and I don't know nearly as much as the other folks responding, my experience with my 10 foot deep 20x40 pool is that I have never been able to get bubbles coming from the main pool drain ...I can leave a 6 hp shop vac hooked up and running for 30 minutes and still don't get bubbles. What I have done for the 7 years since we built it is to let the vac run for while to hopefully push some of the water back down to the main drain and then while it is running, I close the valve and hopefully airlock it somewhat ...this has been ok so far with no horrible consequences ...at least that I could tell.
I have an equipment room that is on the end of a lanai and fully protected ...pipes go straight down for a foot or so through concrete then run sloping to the end of the pool about 40 feet away.
If you are far north with colder climate my process may not be sufficient ...or if lines run more shallow.
One thing I am curious about is what net effect keeping the pump and valves warm would have ...was considering putting a halogen bulb in a reflector fixture and leaving it near and pointed at the equipment this year as extra precaution during extreme cold periods.