I often experience issues with my SB4051-C0 to my 2013 Panasonic S60 plasma. It seems like a handshake issue to me. If I cycle through the sound bar inputs, and it may take more than one cycle, it usually begins working. But it is frustrating that you even have to bother.
Thing is, I have the sound bar volume programmed to be controlled using the TV remote, so sometimes when I first turn on the TV I immediately have control without CEC, thus, no volume control indicator appears on the TV. A few minutes later, the TV remote will stop controlling the sound bar. I will cycle through the inputs on the sound bar using the Vizio remote and try again with the TV remote until the TV remote begins controlling the sound bar volume, which is sent through CEC and a volume up/down indicator appears on the TV whenever I change the volume.
I've written to Vizio about this, but so far there has been no firmware update to address it. However, I have also read that CEC has issues, so I chalk this up as one of those issues. Best bet is get a universal remote or keep your Vizio sound bar remote handily in arm's reach.