A 12 gallon water capacity propane tank holds 9.6 gallons of liquid propane. That "vent" is not really a "vent" at all, but rather it's the "Fixed Liquid Level Gauge" that's used to tell when the liquid has reached the 80% full point, the maximum the tank can legally hold. The OPD (Overfill Prevention Device) will also cut off the refill flow at about the same point as a backup. If the Fixed Liquid Level Gauge valve cannot be opened, a trained refiller should know not to fill the tank. A local propane distributor may be able to correct the problem for you at a nominal cost. The valves are replaceable.