Who is the Minimum Damage on Silver Bolts for?
At rank 1, you'd need a target with under 700 maximum health for the minimum damage to exceed the %health damage.
At rank 5, you'd need a target with under 1200 maximum health for it to exceed the %health damage. Possible against some other ADCs, maybe, but it's not going to kick in most of the time.
Most champions cannot avoid having maximum health that low by the time Vayne maxes Silver Bolts. Right now, the minimum damage only exists to assassinate champions that were AFK for a while.