Not sure that's possible. User profiles function very similarly to wholly separate devices in order to more completely sandbox them from one another. There are a few android settings that are global across all profiles on a GOS device, and some of those are only manageable by the Owner profile, but VPN isn't one of them.
This is a very good thing, overall, but it does have the unfortunate side effect that each profile takes up a device slot in Mullvad.