Fuel filler neck flap doesn't close completely.

You best replace the filler neck so it wont leak, and operate properly. I do not think a vacuum, vapor lock has occurred.

if the flap is intact you can try spraying some wd-40 on its hinge. If its missing you need a filler neck for your fuel tank.

OPEN GAS CAP FLAP THEN YOU NEED TO DRAIN GAS BELOW FILLER NECK.THEN LOOSEN REMOVE SCREWS IN THE GAS CAP FLAP AREA WHERE FILLER TUBE CONNECTS THEN GO UNDERNEATH VAN LOOSEN REMOVE HOSE CLAMPS AND REMOVE THE HOSE AT THE FILLER NECK BELOW THE GAS TANK.SPRAY WD40 AROUND HOSE AREA AT METAL FILLER TUBE STICK OUT GAS TANK.CAREFULLY USE LARGE PAIR OF CHANNEL LOCK PLIERS TO REMOVE WORK HOSE OFF METAL TUBE FROM GAS TANK. IF YOU CANT REMOVE HOSE WITH HANDS.BECAREFUL NOT TO MAKE ANY SPARKS.PLACE RAGS AROUND ANY METAL PARTS TO KEEP CHANNEL LOCKS FROM HITTING METAL MAKING SPARKS.

Sounds like you have something stuck in your gas tank maybe someone put something in your gas tank perhaps you might consider a good flash light and a pen open the cap insert the pen hold tight and put flash light in filler neck flap hole to see what kind of obstacle you have stuck in your filler nozzle neck if your unable to see anything you might consider taking the car to certified technician for service to you clogged gas filler neck.

There is a check valve in the filler hose to stop fuel from coming out seems yours might be stuck , run a metal coat hangers inside the filler neck while prying open the little fill flap. see if that helps OR you are not or have no VENT to AIR in your evap system causing excessive tank pressure.

just behind the fuel flap

The new design filler neck on newer gm vehicles don't have a flap. The flap was just to keep stuff from falling in. I've had problems with the fuel sending unit in the tank on these vehicles.

What you will have to do is remove the upper filler neck from the vehicle where it connects to the body.(where the fuel door is)Should only be 2 or 3 screws.Lower it down and remove it from the hose.There might be 2 hoses.One small and one big.You can either get a new one from the dealership or go to the salvage yard and get a used one.Good luck.

Is this happening at all gas pumps, or just a particular pump?

It could be that your fill hose may be partially blocked, your anti-siphon flap is getting stuck

Replace the filler neck that goes to the tank.