A drinking vessel, comprising: a container having an open mouth and an internal cavity; a removable lid attachable to the mouth, the removable lid having at least one opening; a cover member disposed on the removable lid and moveable between open and closed positions, the cover member having a cam portion; at least one flexible ventilation member that sealingly engages the at least one opening in the removable lid, has an air passage extending therethrough and comprises opposed flexible first and second venting portions (39a, 39b) angularly and downwardly disposed within the air passage and the external end of said air passage forms a vent opening, wherein the ventilation member is configured to allow an inflow of air when pressure within the container enters a negative pressure state in relation to pressure outside the container, and configured to prevent a liquid contained in the internal cavity from leaking therethrough; a vent actuator operably coupled to the at least one ventilation member and the venting portions thereof, wherein the cam portion is configured to contact the vent actuator during movement of the cover member for changing a state of the ventilation member between a ventilation state and a closed state; wherein the vent actuator is integral with one of: a. the at least one flexible ventilation member, or b. the removable lid wherein the vent actuator and the removable lid are attached by way of a flexible bridging portion; and wherein the cam portion is configured to cause a downward and inward movement of the vent actuator relative to the vent opening of the ventilation member when the cover member is moved from the closed position to the open position, the vent actuator causing the first venting portion to move laterally or outwardly away from the second venting portion to bring the ventilation member into the ventilation state. a container having an open mouth and an internal cavity; a removable lid attachable to the mouth, the removable lid having at least one opening; a cover member disposed on the removable lid and moveable between open and closed positions, the cover member having a cam portion; at least one flexible ventilation member that sealingly engages the at least one opening in the removable lid, has an air passage extending therethrough and comprises opposed flexible first and second venting portions (39a, 39b) angularly and downwardly disposed within the air passage and the external end of said air passage forms a vent opening, wherein the ventilation member is configured to allow an inflow of air when pressure within the container enters a negative pressure state in relation to pressure outside the container, and configured to prevent a liquid contained in the internal cavity from leaking therethrough; a vent actuator operably coupled to the at least one ventilation member and the venting portions thereof, wherein the cam portion is configured to contact the vent actuator during movement of the cover member for changing a state of the ventilation member between a ventilation state and a closed state; wherein the vent actuator is integral with one of: a. the at least one flexible ventilation member, or b. the removable lid wherein the vent actuator and the removable lid are attached by way of a flexible bridging portion; and a. the at least one flexible ventilation member, or b. the removable lid wherein the vent actuator and the removable lid are attached by way of a flexible bridging portion; and wherein the cam portion is configured to cause a downward and inward movement of the vent actuator relative to the vent opening of the ventilation member when the cover member is moved from the closed position to the open position, the vent actuator causing the first venting portion to move laterally or outwardly away from the second venting portion to bring the ventilation member into the ventilation state.