I move a unit to a battlefield. The opponent reacts with Rengar Trophy Hunter. I play Vi Peacekeeper. Does Vi stun?

Yes, Vi stuns.

You moved into an occupied battlefield, making you the Attacker. The opponent then plays Rengar via Ambush (as Defender). When you respond by playing Vi, Peacekeeper via Ambush, she enters the ongoing combat and is designated as an attacker — this triggers her "When I attack" ability, letting you stun an enemy unit at that battlefield.

This is directly supported by FAQ #10034 and FAQ #10437: units entering mid-combat gain the Attacker or Defender designation upon entry based on which side played them, and Vi's trigger fires exactly once when she first gains that designation. As long as she's still on the battlefield when her trigger resolves (not removed by a reaction), the stun goes through.