It looks like its lifted off the lid a little bit, instead of cutting it off or beating on it with a hammer...try CPR.
Place the lid on a solid surface like a table top or wood work bench. (Some place that won't rash the rim especially)
Put both hands over the vent and give it a push down, gently at first then push harder if the metal don't flex the first time.
Test the vent after every pump.