Vayne's current damage against a 1000 HP target:
- Rank 1 Silver Bolts - 60 (20 flat + 40 as 4% of 1000)
- Rank 5 Silver Bolts - 140 (60 flat + 80 as 8% of 1000)
Vayne's damage after the 5.22 change:
- Rank 1 Silver Bolts - 60 (6% of 1000)
- Rank 5 Silver Bolts - 120 (12% of 1000, also the minimum value for rank 5 Silver Bolts)
So, against a 1000 health target these changes are a slight nerf to the rank up value of Silver Bolts. At 2000 health rank 1 damage goes from 100 to 120, rank 5 damage goes from 220 to 240, so in both cases slight buffs. At present however Vayne's also got a reduction of 1 AD per level, so at level 9 (assuming she maxed W first for sake of discussion) she'll have missed out on 27 physical damage from those three attacks. Any attacks that don't contribute to a Silver Bolts proc will also be weaker of course too, without the benefit of the extra Silver Bolts damage against a target that's itemized at least some health.
Finally, to look at a later game scenario, let's look at a level 15 Vayne with rank 5 W against a target with 3000 health. Previously she'd have dealt 300 damage per Silver Bolts Proc. With these changes she'd deal 360, though with at least 45 less physical damage as a partial offset.
TLDR: This change is neutral or a nerf against low health targets and a small buff, though one partially offset by lower AD/level, against health stacking targets.