Which Vizio model do you have?
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.