i react with peacekeeper vi during enemy's turn, im defending a battlefield, does peacekeeper vi stun work?

No — Vi won't stun in that case. Her ability is an Attack Trigger ("When I attack"), which only fires when she gains the Attacker designation (Rule 383.4.e). Played by the defending player, she enters the ongoing combat on your side and gains the Defender designation instead (Rule 323.2.a), so the trigger condition is never met.

To be clear on the comparison: the "yes" rulings for Vi (FAQ #10034, #10437) all involve the unit being Ambushed in by the attacker. A unit joining mid-combat takes the designation matching its controller's side, so the same play from the defender side produces a defender, not an attacker.

You can legally Ambush her there (you control the battlefield you're defending, satisfying Ambush's "battlefield where you control units" requirement, Rule 822.1.b), she just won't stun anything.