I realise this is late, but just incase it is still relevant, I think you will find these pages helpful.
You can try and create the VT session passing this parameter : kVTVideoDecoderSpecification_EnableHardwareAcceleratedVideoDecoder () which will enable the hardware acceleration if it is available. You can then poll the session on whether it is using the hardware acceleration by querying for kVTDecompressionPropertyKey_UsingHardwareAcceleratedVideoDecode. Alternatively, request hardware acceleration by passing kVTVideoDecoderSpecification_RequireHardwareAcceleratedVideoDecoder and if it fails, revert to software decoding. Presumably similar options are available for encoding!
Good luck & all the best,