It may not be possible to stream Pandora to an external device via the digital optical port. Checking in the manual is the first place to start, then at Vizio support sites. I can't look in the manual though until you give me a model number ;).
If it is allowed by the manufacturer, then you would do it the same way you have been- via digital optical out on the Vizio, to digital optical in on the Denon receiver.
I have a DENON AVR-1913.
I also have a VIZIO web-enabled TV.
In order to hear audio from TV web apps (Netflix, Amazon, Hulu, Pandora) over the sound system, what connection do I need to make?
Currently I have an optical cable that allows me to hear the audio signal out of the TV only.