Shoryuken has it on knockdown because that's how Vortexes are in 2d games with no block button.
In SF4 or KoF13 for example, a lot of vortexes were from characters that had a good crossup and can combo into a move that leads to a hard knockdown.
Eg: Akuma's sweep, you land it and go for left/right off of a jump-in or demon flip, if they get hit you can link into sweep from s.lp or off of lk tatsu and they're back in the same situation again.
You have options to get out, be it focus, armor, invincible moves or whatever but OS, safe jumps and Akuma's air angles avoided or beat a lot of those options so in the end you're stuck guessing with most characters.
In NRS games, vortexes come from restand moves, be it raw (old Trance+ bat setups) or off of a normal/string that's plus enough to guarantee an OH/low, having one (or both) options being guaranteed (as in no gap to armor/backdash etc) is simply a plus to make it even better, it isn't a neccesity for the mixup/setup to be a "vortex". If it's fuzzy-able it's still a vortex, just not a good one.
If you land the OH or low you'll still be able to get the restand and go back to the same scenario, hence the term "vortex".
If they can armor that means they'll need to spend a bar and (usually) take a risk to try to avoid being in the vortex. Don't want to take the risk then use it against a character with no armor or when they're meterless.