I found the following regarding users being logged out of vbulletin forums...
Your login status relies on cookies stored by the browser on your device. Logins will not be shared across devices or browsers. Logging out on one device will not log you out of others.
You can be logged out if -
- You block cookies from the server.
- Cookies are deleted from the device,
- You have not selected "Remember Me" on the front-end and your site is behind a proxy (i.e. Cloudflare) and the reported IP Address changes.
- If you are inactive longer than the Cookie Timeout value and you have not clicked "Remember Me". This is 15 minutes on a default server.
- You block JavaScript for any reason.
make sure the user isn't blocking cookies or Javascript on your site. If they block either they will get logged out.