There should still be a degree to which she can riposte though.
I still disagree. It's not a necessary change and there are better ways to balance her. Requiring her to reshift the exact direction she's facing for the use of a spell isn't very interesting. It wouldn't even work very well because the game has instant turn speeds. I could see this working in Dota, but if a character can turn instantly, you're just making it jankier to use without adding interesting gameplay.
It would be significantly more frustrating for Fiora than it would be interesting for the enemy, and that's not true for balance changes that don't fuck with her mechanic. As far as I'm concerned, that makes it a bad idea.
Even if we had to hit the W, there are better options. Reduce the range a little. Reduce the width. Reduce or take away the damage and slow unless she blocks a hard CC, increase the cooldown, increase the cooldown if she doesn't parry a hard CC. There are so many alternatives, and all of them are better than a facing-based change to riposte.
it plays into the Vital minigame
Not really. Vitals have an absolute direction, and don't take facing into account. If they did take facing into account, they'd be a terribly designed mechanic. It really has nothing to do at all with vitals, or any part of the mechanic. It doesn't make sense.
giving her that much defensive power all around leaves little room for counter play,
I completely disagree. There's a lot of counterplay to predicting her invulnerability timing, spacing abilities, not grouping up, baiting her W out, forcing it out when she can't stun someone important, etc. You have a ton of ways to limit the effectiveness of her W. It's her overtuned stats that leave little room for counterplay. Hit her stats, make her reliant on hitting a good W, and then we're perfectly fine. She's weaker and we didn't have to try to force a really weird facing-based weakness into a 0-turn-speed game.