Recording vMix to BMD Video Assist 5

I'm trying to record the output from vMix, both audio and video, to my Video Assist. Please excuse my lack of knowledge. I don't know how to make this happen. I can get video by turning on Fullscreen and plugging the Video Assist into an HDMI port on the laptop where I would connect an external monitor, but I don't get audio. What am I doing wrong? Is it possible to do what I want to do?

what is your platform, mac or pc ?
do you have a thunderbolt port on the machine ?
what resolution / framerate are you trying to export to ?