Number 1 counterplay is denying her Vitals, which you do by matching her movements (in fight) as much as you can (and posture/position so that she can't easily Q for one outside of trades). Keep in mind a few things: Her Q is fairly short CD unless she misses everything with it (no hit on anything), when a Vital is proc'd a new one starts to appear almost immediately, it can be collected before the animation of it's appearance is complete, so don't think you're safe just because that animation is still playing.
Staying at max melee range when fighting her usually means she can't Q to the far side of you to get a far-side Vital, which means it stays and she doesn't get a new Vital elsewhere during the trade/all-in.
Why is Vital denying so good v Fiora? Apart from the obvious denying of significant true damage (which counters resists, so denying Vitals makes your armor more effective against her), it also denies her the flat heal and movement speed (which means she will have an even harder time getting to the Vital she needs to pop).
Btw, Fiora's W has a bit of a delay on it, and doesn't last very long, so if you can avoid telegraphing your use of CC, it can often get through even when her W is up.
Immediately after she Q's, even if she gets a Vital off of it, is usually the best to time to engage a trade, as her Q is now on CD and won't be up for a while, especially if she missed everything (no 60% off CD, so it may be as long as 16 seconds!). Be careful though to note if she popped a Vital, where the new one is appearing, you don't wanna jump on her if that will lead to her being on the right side to get that fresh Vital without even having to move.
Lastly, Vitals never appear on the same place twice, so if she just got one from a given side, the next one won't be there, and that side is 'safe' outside of her ult'ing you for a bit. I've noticed that every other Vital seems to appear directly opposite where the last one was, and the others are 90% off. I've also noted that it tends to vary between top-right and bottom-left pairs most of the time in lane (top lane). So that's handy for helping you anticipate when your more exposed Vitals will appear v. safer ones and when to time your trades.