I'd prefer it the other way around, reduce the flat damage, add a bit more %, and add a bit fat truedamage on her ult instead of bons ad.
Vayne with crit just obliterates squishys with her good base stats and the 70 ad steroid from ult.
I think vaynes niche is countering bruisers, beeing a shortranged tankbuster.
For seastroying squishys on 550 range, we already have draven. He and Vayne both have little effeckt above their 550 range(in contrast to any other adc), and both make up for that by massive damage. Currently there only realy difference in terms of niche is that vayne is more late focused, while Draven is more early.
However, i feel like it would be good to differ their niche even more, haveing draven with raw damage to kill squishys while vayne exels at shredding tanks.
Curretnly, vayne will blow up squishys and shred tanks alike if she gets her items, beeing simply a better draven for anyithing but the lane...