You can use Apple Configurator 2 to assign VPP apps to iOS 9 and later devices. If you choose an app that doesn’t allow device-based assignment, you see a prohibitory symbol () on the app icon.
If you select the app, you see a “This app cannot be assigned to a device" alert message.
You also see this message if you move your pointer over the app name in the Choose Apps sheet in Configurator.
Before you buy a VPP app that you want to assign to iOS 9 devices, look for “Device Assignable” in its description. This confirms that you can use device-based enrollment for the app. If “Device Assignable” isn't in the app's description, click "[App Name] Support to contact the app developer and request that they turn on device-based enrollment for their app.