I move to a battlefield with a unit unsung hero, and i ambush with peacekeeper vi, can enemy react with overzealous fan first before i can ambush with my vi

No — the Fan can't beat you to it. Fan's ability isn't a reaction it can choose to fire at you; it's a "when I defend" trigger that fires automatically the moment it gains the Defender designation, and it goes on the combat chain. You hold Focus as the attacker, so once it's on the chain you get priority and can Ambush Vi at Reaction speed (FAQ #10691).

One catch that matters here: Fan's controller has to kill the Fan as the cost and lock in their target the instant they place that trigger (Rule 355.5.b) — and Vi isn't on the board yet, so they can only name your Unsung Hero, not her. When Vi lands she gains the Attacker designation and her "when I attack" trigger goes on the chain above Fan's, so her stun resolves first and Fan then bounces whatever it locked in.

This is the reverse of FAQ #11389, where Vi is the unit that moves in — there both triggers fire together and Fan sits on top, bouncing her before she can stun. (If the Fan were the opponent's only unit there, killing it as the cost leaves no enemy unit for Vi's stun to target, so her trigger would do nothing.)