Your VacuGlide cap needs one of these silicone valves to work.

It's easily removed by inserting a flathead screwdriver underneath and gently pushing upward. CLICK HERE to watch a video on replacing and removing the valve. If you try to remove it with pliers or otherwise try to remove it without gently prying it loose by the edge, it will tear and will need to be replaced. Each set contains two valves, so you'll always have an extra if needed.