The answer is it works. I picked up the VHT215 at BJ's - took the HDMI cable out of the TV and plugged it into the soundbar's HDMI input 1. Then I plugged the HDMI output into the TV - switched off the TV - turned on the soundbar - chose HDMI input 1 from the soundbar's remote - rebooted the PC - when Windows Media Center started the soundbar was working.
[Update after using it for two days] We went to SPDIF from the computer to the soundbar for the computer - if the TV was turned off and then turned back on we get a message that the TV doesn't support HDCP. With the SPDIF connection everything works exactly the same way and sounds as good if not better (not sure how a different digital signal would sound better - maybe its something about the way the vizio inputs work).
I can control volume up/down/mute with the Media Center remote. I set the soundbar at 23 out of 30 and put it's remote away.
The manual says you need ARC - perhaps if you want advanced adjustments for audio on the TV menus to work back into the soundbar you would need ARC but to just control volume the HDMI connections work fine.
The VHT215 sounds very good in our 12x12 room with the TV in the corner.