To help with Vitals just remember this : You do NOT have to directly hit it. So if the vital spawns on their back, Q to their side at an angle and you will hit it. You don't have to directly Q into it. That's what people use to bait Fioras in, they know you are going for vitals and they know you have to hit it at a certain angle. So as long as you know what angles will proc that vital, then you should be good and not get baited to your doom.
For your Ult, it's rather simple really. Weaving attacks. Ult, Auto the first one. Q to the side and hit the second one. Use E and Q again to a side and hit the third one. By this time they will try to run so you don't hit that fourth one, but your Fiora, you can Q over and over as long as you hit something with it. Q again and finish off with that last E auto. Sometimes i take exhaust on Fiora just to be an ass lmao. But anyway, if you can weave autos with your E and Q and cancel animations you won't have any problem with that.
Like Riven, Fiora is a very good champ to weave autos and animation cancels with. It should only take you a matter of seconds to hit 2 vitals of her ult. Ult, Auto. Boom first one down. Q to one side. Boom second one down. Flash E auto. Boom third one down. Then Q and get that last E crit in there and take out the last one.
Practice animation cancels and weaving attacks in between them and it'll help you a lot with her ult. And then just learn the angles of which you can hit Vitals with a Q, and you should be good.