If the Vault app closes unexpectedly after using Face ID, follow these steps to troubleshoot and resolve the issue:

1. Check for Updates: Ensure that both the Vault app and your device's operating system (iOS) are updated to the latest versions. Manufacturers often release updates to fix known bugs.

2. Reinstall the App: Uninstall the Vault app and then reinstall it from the App Store. This can resolve issues caused by corrupted data or improper application installation.

3. Clear Cache: If applicable, try clearing the cache for the Vault app. This is done by going through the app settings in your device's settings menu.

4. Re-enable Face ID Settings: Go into the settings app on your device, locate the Vault app, and toggle the Face ID access off, then back on. This may help reset any issues related to the biometric unlock feature.

5. Test Without Face ID: Try logging into the app using your master password without using Face ID. If the app remains stable, the issue may specifically relate to Face ID interactions.

6. Report the Issue: If the problem persists, consider reporting it to the app's support team or checking community forums. Other users might be experiencing similar issues, and they may share their fixes or official responses.

7. Consult Known Issues: Review threads and reports from other users regarding Face ID and crashing issues. For instance, discussions in forums like Bitwarden and GitHub detail multiple users facing crashes after using Face ID or when attempting to log in. These reports indicate that uninstalling and reinstalling the app often resolves such issues - learn more - learn more.

8. Contact Apple Support: If after all these steps the issue continues, it may be worthwhile to contact Apple Support for assistance. There may be a device-specific issue that requires professional support.

By systematically following these steps, you can address the problem of the Vault app closing after using Face ID successfully.