GrapheneOS requires a locked bootloader and supports using deveice attestation via the generic attestation functionality in the Android Open Source Project.
Play integrity is an anticompetitive tool that ignores this, and artificially limits itself on GrapheneOS. It is not due to any incompatibility.
The VW app can't do remote unlock so that's not a problem. It allows you to turn on the aircon or start charging and that's about it. That only works 50% of the time anyway.