For Vi Legends, if I take with a 2 might unit and a 3 might unit into a 2 might unit, have I dealt 3 excess damage?

Yes, you've assigned 3 excess damage.

Here's the breakdown:

• Total attacking Might: 2 + 3 = 5
• Single defending unit: 2 Might
• Lethal damage for a 2 Might unit = 2 damage
• Per Rule 460.2.c.4, since there are no further defending units after that one, you can assign the remaining damage to it: 5 total assigned
• Excess damage = total assigned − lethal = 5 − 2 = 3

FAQ #11256 confirms: "Excess damage is the amount of damage assigned to a unit that goes beyond the amount required to kill it (lethal damage)."

So when you conquer that battlefield, Piltover Enforcer will see that you assigned 3 excess damage — meeting its "3 or more" threshold — and you may exhaust it to ready a unit.