If your custom options are resetting on launch, it's likely because your user_settings.config is set to 'Read-only'. This can be changed by:
- Press the Windows key + R
- Enter %appdata% and select the ‘OK’ button
- Navigate to AppData\Roaming\Fatshark\Vermintide 2
- Right-click user_settings.config and select ‘Properties’
- Under ‘Attributes’, deselect the ‘Read-only’ checkbox
- Apply the changes
If this isn't the case, try setting ‘Graphics Quality’ to ‘Custom’ to start with, and then apply your desired changes and click ‘Apply’.