The stamina reduction from vindication will reduce a mob's maximum health, but will not reduce current health unless it is more than the new maximum.
Vindication always procs from [Hand of Reckoning]. This means hand of reckoning can be used to consistently apply the vindication debuff before damaging a mob, gaining the greatest possible benefit from Vindication's effect.
If vindication reduces a Mob's maximum health below its current health, the mob will reward reduced experience. The experience rewarded is (maximum health after vindication) / (health before vindication) * 100%.
This talent is mostly useful in PvP and solo grinding, as almost all bosses in raid instances are immune to this debuff.