You say prefix, but do you mean suffix? The suffixes suggest that "vindict ive " is likely to be an adjective (not a noun as you imply), and that "vindic ate " is likely to be a verb. Other than that, they provide no clues about meaning.
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Thomas Tompion2. That vindicate comes from vim dicere, the Latin for to speak with force - and has come to mean to justify.Do you have a source for that? This is not what it says in any dictionary that I can find.