When Should I use vi's ult?

You use it as a last resort in ganks; only if you need it to grab the kill, which you usually won't.

If it gets you the kill, go ahead and use it.

I'm just saying in teamfights save it for nullifying CC or after an important escape is used by your target. If you initiate with it, you're left as a sitting duck in the middle of the enemy team.

While that red line is there and you are charging towards the poor innocent adc about to get two shot by you, you can't be disabled.

Once you land from smashing them into the sky, then you can be CC'd as normal.