Hyundai and Kia added official GrapheneOS support to their apps months before Volkswagen banned GrapheneOS:

https://discuss.grapheneos.org/d/31697-myhyundai-app-has-official-grapheneos-support

Pressure from Volkswagen customers on them can achieve the same thing. There's no legitimate reason to ban GrapheneOS so they'll undo it with pressure.

myHyundai app has official GrapheneOS support - GrapheneOS Discussion Forum

GrapheneOS discussion forum

GrapheneOS Discussion Forum
Leave a 1 star review for Volkswagen's apps on the Play Store asking them to stop banning GrapheneOS. Explain it's a far more secure operating system and fully possible for them to verify the hardware, OS and their app on it if they insist on doing it. It's far more secure than anything they allow.
Google has misled companies about what the Play Integrity API provides. It doesn't genuinely enforce having a secure device or legitimate app, it only pretends to. It leaves huge security holes open. It enforces Google's business interests and bans having a reasonably secure device with GrapheneOS.
@GrapheneOS
More generally, the point of this kind of attestation in the general public is always to enforce developer's interests *against* the user. Often also anti-competitive. Even if they supported GrapheneOS at some point, general OS development would still be blocked. And even you as GrapheneOS would make yourselves dependent: They can always threaten you to withdraw attestation if they don't like your future plans for even better privacy protection against their abuse.