If I Charm an opponent's unit into my controlled battlefield may I use Vi Peacekeeper's ambush to stun the Charmed unit?

Yes, you can use Vi, Peacekeeper's Ambush ability to stun the unit you just moved with Charm, provided you meet the requirements for Ambush.

Sequence of Events

1. Charm: You play Charm, targeting an opponent's unit and moving it to a battlefield you control.
2. Forced Showdown: Per RiftJudge FAQ #7385 and #9843, moving an enemy unit to a battlefield you occupy forces a showdown, and the opponent who controls the moved unit becomes the attacker.
3. Ambush as Reaction: Because you have a unit at that battlefield, you satisfy the requirement for Vi, Peacekeeper's Ambush keyword. Even though the showdown has begun and the state is "Closed" (if the move or another effect put an item on the chain, or simply by virtue of the showdown rules), you may play Vi as a [Reaction].
4. Entry and Designation: Per RiftJudge FAQ #10034, when Vi enters the board during a showdown, she joins the ongoing combat and is designated as an attacker.
5. Trigger: Gaining the "Attacker" designation fulfills her "When I attack" ability.
6. Resolution:
   • Vi's ability is placed on the chain.
   • Provided Vi remains on the battlefield ("here"), her ability resolves, and you may choose an enemy unit at that battlefield—in this case, the one you just moved with Charm—to be stunned.

Important Considerations:

• "Here" Constraint: Per RiftJudge FAQ #10045, Vi, Peacekeeper must remain at the battlefield when her ability resolves. If your opponent plays a card to remove her from the battlefield before her trigger resolves, the ability will fail to find its source and will not stun the unit.
• Stunned Status: As per Rule 423.1.a.1, a unit that is already stunned cannot be stunned again.
• Timing: This ability triggers exactly once when she gains the Attacker designation upon entry (FAQ #10205).