If I am attempting to conquer a battlefield currently held by my opponent who has a 3 might and a 2 might unit on it, I move a 2 might unit to the battlefield then ambush in Vi Peacekeeper. Since Vi has 5 might, can she attack both units and stun them both?

No, Vi, Peacekeeper cannot attack both units, nor can she stun both of them.

Why Vi cannot stun both units

According to her card text and RiftJudge FAQ #10205, Vi's ability is a triggered ability that happens exactly once:

• Trigger Condition: "When I attack, [Stun] an enemy unit here."
• Single Effect: This ability triggers only when she gains the "Attacker" designation upon entering the combat. It carries out one instruction: to stun one enemy unit. It does not contain "Repeat" or any mechanic to target multiple units.
• Result: You may choose one of your opponent's units (the 3 Might or the 2 Might unit) to stun. The other unit remains unaffected by her ability.

Why Vi cannot "attack both units"

In Riftbound, units do not "attack" individual enemy units. Instead:

1. Combat Designation: Vi enters the battlefield and is designated as an Attacker.
2. Combat Damage Step: Combat damage is not assigned to individual units by the attacker in a 1-to-1 manner. As per Rule 460.2, the total Might of all attacking units is summed, and the total Might of all defending units is summed.
3. Damage Assignment: Starting with the attacker, players assign their total Might as damage to the opposing units. You are required to assign lethal damage to a unit before moving to the next.

In your scenario:

• You have Vi (5 Might) and your other unit (2 Might), for a total of 7 Attack Might.
• Your opponent has units with 3 and 2 Might (total 5 Defense Might).
• During the Combat Damage Step, you will assign your 7 total Might as damage. You must assign at least 3 damage to the 3-Might unit and 2 damage to the 2-Might unit to kill them (if they are not otherwise protected).

Vi will participate in this combat alongside your other unit to contribute her 5 Might toward that total, but her specific Stun ability is a one-time effect that can only be used on a single target.

This is my interpretation based on the standard rules for combat and triggered abilities. No official FAQ exists for the specific interaction of Vi's stun vs. multiple units simultaneously, but the rules for "When I attack" triggers (FAQ #10205) confirm the ability only executes once.