With VGT, it becomes the guest operating system's responsibility to assign VLAN tags to its outbound traffic. The port group to which such a virtual machine is connected should be configured with VLANID 4095 (meaning to trunk):

In VGT mode, the VLAN-tagged traffic will flow unmodified through the virtual switch and to the physical switch. The guest operating system is solely responsible for tagging and untagging on the frames.