A question perhaps for a plumber?

I don't think introducing an air vent would solve your problem. It appears that when the pool overflows there is more water going into the pool than can get out via the 6" pipe. The answer may be to increase the diameter of the 6" pipe or install an additional one. Or, you could install a land drain (say 4") around the top side of the pool to reduce the amount of surface water going into the pool during the wet season, this land drain you could also run to the ditch.