This is correct if you revoke the VPP assignment from the user. However if you are also using Managed apps in Self Service and unscope it the app will immediately uninstall.

Sorry to reanimate a dead thread.

If I understand this correctly, the 28-day grace period only applies to user-based assignment?

If I unscope an app using device-based assignment, the app will delete immediately?

Regardless of how the license is granted, if you unscope a managed app, the app will be deleted for that device unless the device is in scope for a different application object for the same app. (This happens with user-based VPP due even if the license is still assigned to that user.) What I wonder but have not tested is if/how long the app will stay available if you have a second object for the same app and do NOT check the Assign VPP Content box (so no DEVICE licenses are granted). If you have the device scoped to the app object that's not granting licenses, it will still be in scope even after unscoping it only from the object with licenses so JAMF will not automatically uninstal the app. In theory, I would think you'll get the VPP device license back to resuse since you are no longer granting a device license via any app object, but I don't know if that would have a time limit like user based-VPP.

Somewhat releated is that I have found if I delete a device with VPP device licensed apps without unmanaging it, the apps stay on the iPad but they never get updated with new versions. This makes to me since JAMF is no longer communicating with the device so it's neither removing apps nor managing updates. While I'm not sure, I believe VPP device apps can only be updated through JAMF so that's why they stay at the version they were at when deleted from JAMF as opposed to being updated by the App Store.