GOS-user while apps are separated via profiles, you still need to use the same app version on each profile to avoid the conflict you're describing. I assume you try to install ProtonVPN from a different source (e.g. Play Store vs. F-Droid) or an older version of the one already installed in another profile.
If ProtonVPN is already installed on your owner profile, you can simply push it to the desired user profiles in the profile settings ("Install available apps"). This way you could for example use the owner profile for the sole purpose of installing and updating all your apps and push them to their related user profiles where you actually use them and feed them the required data.