Diagnosis
The remote for this model pairs via Bluetooth, not infrared. It can lose its pairing with the TV, or the TV's Bluetooth receiver can glitch.
How to Fix It
First, restart the TV by unplugging it from the wall for 60 seconds. Plug it back in. Once it's booted, take the batteries out of the remote. Press and hold every button on the remote for 5 seconds to discharge it. Put the batteries back in. Now, point the remote at the TV and press and hold the 'Menu' and 'Volume Down' buttons together for 5 seconds. You should see a pairing notification on the screen.
If That Didn't Work
You can use the Vizio SmartCast mobile app as a full remote in the meantime. Also, check if the physical buttons on the back of the TV itself work. If they do, the problem is isolated to the remote or its pairing. If the TV's buttons are also unresponsive, the TV's main board might have a deeper issue.
Heads Up
Other Bluetooth devices in your home—like speakers, headphones, or even a nearby laptop—can sometimes interfere with the remote's signal. Try turning other Bluetooth devices off temporarily to see if the remote starts working again.