Shen and Gragas can flash while dashing and get their CC

The timing is different for Vi. With Gragas and Shen, you would want to use the flash at the end of the ability to extend your ability to it's maximum. With Vi, its near the middle of the launch of her Q. and you get that extra flash range before your ability travels to it's end mark. Check out ScrapComputer's video here: https://www.youtube.com/watch?v=QGmKXpP-wtw

It covers flash extensions and animation cancelling with flash covering plenty of champions and abilities. It's all about getting the timing right on champs' abilities. Vi's is a little trickier than Gragas, Shen, or Sejuani as you have to flash earlier than these examples but just practice it and you can get it.

VI can, i did it often enough to be sure, and so can J4 duering his Q+E combo...

Vi can.

Pretty sure she can.

There are A LOT of things in this game that are weird. This is one and it can be done. It's the main reason we want the sandbox mode. I'll never learn the alistar mo(o)ve without it.