A number of 19th-century textbooks suggest "avenger". For example, Thomas Swinburne Carr writes:
Vindice, avenger. So the minister of justice was termed.
Writing in Latin, the 17th-century commentator Daniel Crespin suggests:
Vindice.] Punitore. Nulli enim tum erant judices, nulli lictores, nulli rerum capitalium vindices, nec ulli carceres.
Crespin uses ultor in a prose rewrite of the sentence:
Prima facta sunt tempora aurea, quæ ultro sine lege ultore nullo servabant fidem et æquum.
Maybe Peter Burman the Elder wrote this:
Vindice nullo] Cum nullus esset, qui mulctam aut pœnam aliis irrogaret, si forte deliquissent.
I gather from all this that a vindex was someone with authority to order a remedy or penalty in a law suit, but unlike iudex, the word evokes notions of vengeance and vindication—English words that derive from its close relatives in Latin. Lewis & Short suggests "avenger, punisher, revenger".
The more of these commentaries I read, the less sure I am of who wrote them, but this rather idiosyncratic one:
Vindice nullo, nemine scelera coercente
suggests to me someone like a sheriff, who keeps criminals "in line" by a credible threat of force—not needed in the Golden Age, since people then were loyal and upright sponte sua. Since the Golden Age itself, not the people, is the grammatical subject of the sentence, and "protector" in English carries the unwanted suggestion that the Golden Age might have needed protection, this suggests a liberal but accurate translation of vindice nullo that might do justice to all of the above: without coercion.