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.

Most companies are unlikely to stop using the Play Integrity API but most are willing to start permitting GrapheneOS via hardware attestation with enough pressure.

In addition to every user of their app on GrapheneOS leaving a 1 star review on the Play Store, multiple other steps can be taken too.

Every GrapheneOS user with one of their cars using the app should file a customer support request. Keep answering them and countering the template responses. Escalate the request higher up. Tell them you want money back for the vehicle due to reduced functionality after the fact and insist on it.