I've chased this exact complaint on more sets than I can count, and the good news is that a Vizio shutting itself off is usually a setting or a signal, not a dead TV. The trick is to stop guessing and rule things out in the order they actually fail. Work down this list and most people find the culprit in the first three steps.
Start with the timers, because they're the usual suspect
Before you assume anything's broken, check whether the TV is just doing what it was told. Vizio sets ship with a Sleep Timer and an Auto Power Off feature, and the latter is the sneaky one. Auto Power Off shuts the screen down after a stretch with no signal, and a lot of folks trip it without realizing.
Hit the Menu or the gear button, go to Timers (on SmartCast sets, look under System), and set Sleep Timer to Off and Auto Power Off to Off. Watch it for an evening. If the shutdowns stop, you were done before you started.
Next, suspect an HDMI device telling it to sleep
This is the one that fools people. Vizio's CEC feature, which Vizio calls a few different names depending on the year, lets connected gear send power commands over HDMI. A soundbar, a cable box, or a streaming stick that drops into standby can hand the TV a shutdown signal, and the TV obeys.
Go to System, then CEC, and switch it Off. Then watch the set. If the random shutdowns die with CEC off, you found it. You can leave CEC off for good, or turn it back on and instead change the standby behavior on the device that was triggering it. I usually just leave it off and use the TV remote.
Then look hard at the power feeding it
Vizio power boards are picky about clean voltage. If the TV does a clean shutdown and then comes right back, or it only acts up at certain times of day, the wall power is a prime suspect. Plug the set straight into a known-good outlet, skipping the power strip for a test. If that steadies it, your old strip or a marginal circuit was the problem, and a decent surge protector is the permanent fix.
While you're back there, do a real power cycle. Pull the plug, wait a full 60 seconds, hold the TV's physical power button for about 10 seconds to drain the board, then plug back in. That clears a stuck state more often than you'd expect.
Rule out a firmware or SmartCast hiccup
If timers, CEC, and power all check out, a buggy firmware build or a crashing SmartCast app can force restarts. Connect the TV to Wi-Fi, go to System and then Check for Updates, and let it install whatever's pending. If a specific app is crashing the set when you launch it, a factory-style reset of SmartCast (System, Reset and Admin) clears corrupted app data. Save that for after the easy stuff.
When it's heat, not software
A set crammed into a tight cabinet or sitting under a sunny window will overheat and shut down to protect itself, usually after it's been on a while rather than the moment you turn it on. Put your hand near the top vents. If they're hot and dusty, give the TV a few inches of breathing room on every side and blow the vents out. Heat shutdowns that follow a predictable warm-up time are a strong tell.
When it really is the hardware
Here's where experience matters. If you've cleared timers, CEC, power, firmware, and heat, and the set still cuts out, or worse, it clicks and tries to restart in a loop, you're almost certainly looking at the power supply board. Bad capacitors and failing backlight drivers are the classic Vizio failure, and they get worse with time. On an older or budget set, a board swap can cost close to what the TV is worth, so price the repair against a replacement before you commit. On a newer, larger Vizio, a power board replacement by a tech is usually worth it.
Run the list in order and you'll either fix it for free in ten minutes or land on a clear, honest answer about whether it's worth repairing.