Okay this typically indicates a hardware fault, meaning a bad transmitter in the soundbar, or a bad receiver in the sub, both are very common failures on these vizio units.
What we will need to do is reset the system, then re-pair it.
Unplug power to both the soundbar and sub, for at least an hour.
Press and hold power button on soundbar while plugging back in, and for another 30 seconds after.
Then, plug the sub into the same power outlet as the soundbar is using.
Next press and hold the button on the back of the sub for a full 5 seconds, then press and hold the POWER (not bluetooth) button on the soundbar for 5+ seconds, the lights on the front of the bar should flash 3 times.
Let me know if the sub works at that point.