My bottle has a vacuum when I drink

If you are experiencing a vacuum in the bottle after taking a sip it means the small air valve is stuck closed.

1 - Remove the lid and flip it over.

2 - On the underside you will see a small rubber valve, simply manipulate this with your fingers a bit to pop it open.

3- Reinstall your lid and this valve should allow air to flow into the bottle while drinking and prevent any vacuum from occurring.