Diagnosis
Wireless interference is the most likely culprit. The 2.4GHz frequency Bluetooth uses is crowded by Wi-Fi routers, microwaves, and even some wireless subwoofers.
How to Fix It
First, power cycle both devices. Turn off Bluetooth on your phone, then unplug the soundbar for 30 seconds and plug it back in. Re-pair them fresh. More importantly, create space. Move your Wi-Fi router at least 3-4 feet away from the soundbar. If your phone is in your pocket while connected, try keeping it on a table closer to the soundbar to strengthen the signal.
If That Didn't Work
'Forget' the Vizio device in your phone's Bluetooth settings. On the soundbar, go to Menu > Bluetooth > and make sure it's set to 'Visible'. Pair them again as if they're new devices. This clears out any corrupted connection data.
Heads Up
You can't have two Bluetooth devices connected to the soundbar at once. If you paired a tablet previously, it might be trying to reconnect in the background and booting your phone off. Only one at a time.