If Vindicta buys Mystic Burst / Improve Burst, and an assassinate shot kills an enemy due to the extra damage from the shot, the soul bounty is not rewarded.
Intended "Landing a killing blow on a player with Assassinate grants you bonus souls." Both items create separate instance of damage named after item which doesn't count as assassinate's dmg. Talon and MnK for example have special 3s grace period after ult to count kill, Vindicta doesn't.