I have a vizio sound bar model VSB200-NA
http://store.vizio.com/vsb200-na.html. Make sure the following options are set in AppleTV.
Sound Effects = ON, Dolby Digital = Auto/Off, Audio Output = Auto
Not sure if you have the same model, but if you do, be aware that it can only send PCM digital audio over S/PDIF Fiber Optic TOSLINK Input. It will not play AC3 audio. If I set the AppleTV option "Dolby Digital = ON", all i hear is an inital crackle then silence, and the sound bars' LEDs just keep flashing. I only get 2 channel AAC from my m4v encodes. Seeing as it's in my bedroom, it really isn't a big deal. It has great sounding SRS HD processing.
This is a good quick and dirty check of things concerning Vizio sound bars'
http://www.ehow.com/info_10060566_vizio-soundbar-problems.html and this review at
http://soundbar.com/vizio-vsb200/