If you walk far enough away or lose vision of her than the vital will drop, you can then move back to the wave and “force” the vital to be on the opposite side of her. Orb walking with Fiora when she dashes can sometimes help, walking in the same direction as her dash to keep her just out of reach of the vital and not proc it. You can also use dashes to do this, it might actually be easier to do this with dashes lol.
Try and position yourself near a wall, if you have a backside vital (the vitals that are on the opposite side of your model relative to Fiora) then you can go next to the wall to make it nearly impossible for her to proc it. It is very important to do this when she ults you and you are near a wall, don’t waste time running to a wall if you are in the middle of the lane, just focus on fighting her in that case.
Try and hold your most important CC or biggest burst of damage until she uses W, you can also bait W out by faking an engage (using your smaller CC and attacking like you are going to commit to a fight). If your champ doesn’t have a lot of CC, you can use it to bait w out so she can’t avoid your biggest burst of damage.
Fiora is pretty squishy early so you can bully her pretty easily with the correct champ (Pantheon is the best imo but Quinn and vayne are also good) or by playing a certain way, doing small trades while her W is down for example.
And just so you know the vital placement rotates. It rotates between a front vital and a back vital, I forget which side vital is a front or back vital. If Fiora is on blue side (nexus is on bottom left side of the map) then the left side vital is a front vital and the right vital is a back vital, and reverse if Fiora is on red side.
Hope this helps!
Lmk if you need anything else and good luck!