I am defending with a Rift Herald, I cast Sacrifice and then play Vi. Do I get a stun on the attacker? Is Vi now an attacker?

1. Does Vi stun the attacker?

No, you do not get to stun an enemy unit with Vi, Peacekeeper in this scenario.

Vi, Peacekeeper’s ability ("When I attack, [Stun] an enemy unit here") is a triggered ability that requires the unit to possess the "Attacker" designation. In this scenario, you are the Defender. When you play Vi, Peacekeeper via Ambush into an ongoing combat, she joins the combat as a Defender, not an Attacker. Because she does not gain the "Attacker" designation, her "When I attack" trigger condition is never met, and she will not [Stun] an enemy unit.

2. Is Vi now an attacker?

No.

As established in the rules of combat (Rule 459.2.b.2-4), units entering an ongoing combat are designated based on the controller's role in that combat. Since you are the defending player in this showdown, any unit you play into that battlefield joins as a Defender. Because Vi enters as a Defender, she does not trigger her "When I attack" ability.

3. Ability Clarification

In Riftbound, there is a clear distinction between abilities:

• Triggered Abilities: Vi’s ability is a triggered ability. These can occur during a showdown provided the condition (gaining the "Attacker" designation) is met.
• Activated Abilities: Rules regarding Action/Reaction keywords (as seen in FAQ #6091) do not apply to Vi's triggered ability.

Because you are the Defender, you cannot fulfill the "When I attack" condition required to trigger Vi, Peacekeeper.

Sequence of Events

1. Combat State: You are currently the Defender at the contested battlefield.
2. Cast Sacrifice: You play the spell Sacrifice, choosing a friendly [Mighty] unit as the additional cost. That unit is killed, and you draw 2 cards and channel 1 rune exhausted.
3. Play Vi, Peacekeeper: You play Vi, Peacekeeper via Ambush to your battlefield.
4. Designation: Vi enters the battlefield and, because you are the defending player, she receives the Defender designation.
5. Trigger Check: Vi’s ability ("When I attack...") checks for the "Attacker" designation. Since she is a Defender, the condition is not met, and the ability does not go on the chain.

Note: I could not fully verify this ruling against the retrieved rules.