Have you recently updated Data ONTAP? The network cmdlets only work with Data ONTAP 7.3.x. Support for those APIs will come in a later 8.x release, but currently they are not compatible with any 8.x controllers. If your still using Data ONTAP 7.3.6.. then I think you found a bug. we'll look into it, but please report back any additional information.
The good news is that the recently released Data ONTAP 8.0.2 includes the network APIs that were missing from 8.0.1. The APIs are generally backward compatible, but as you pointed out, the net-vlan-create API definition was changed to make the gvrp-enabled field mandatory, so whether or not the change was intentional, we'll have to update the PowerShell Toolkit accordingly. We plan to re-test all of the network cmdlets with 8.0.2 during the current development cycle.