This article only applies to the old version of Opal (on iOS 15 or prior). Here’s why you should upgrade to iOS 16 and experience the new Opal.
Depending on the type of VPN you have, you may be able to use both Opal and the VPN without any trouble. If your VPN provider allows you to change your VPN to a different protocol, switching its protocol (if it doesn't work with Opal simultaneously right now) will often solve the issue! In your phone's Settings, you should see Opal VPN under 'VPN configurations' and your other VPN under 'Personal VPN'.