Locke, as he casts poison on Vaan, Vann casts haste on himself (because locke;s fater and Vann had already picked his move). The poison damages Vaan, he uses an antidote, he wastes a turn, Locke uses poison again etc.
Locke wins.
You're forgetting that Locke could use Reflect. Vaan. He could reflect immobolized and bleed Locke to death.
...Or, Vaan could get multiple hits with a sexy, masumune, killing..them..all!!!