Your question made me do some research (again) and there is much evidence (Vulkan is not needed) that the GPU is not the actual reason why a FP2 with Android 7 Nougat probably wouldn’t pass the Google CTS.
This article on androidauthority.com finally pointed me into another direction: Missing encryption capabilities. The Android 7.1 Compatibility Definition Document (CDD), however, doesn’t differ much from the Android 6.0 CDD in that regard.
At least the CDD for Android Nougat exists by now, so maybe someone else finds any hint to why FP2 may not be able to pass the CTS for Android 7. I still don’t see any reason.