Can Kha'Zix be played in response to Faefolk before Vex arrives?

They must pay Deflect to target Vex. They can respond to Faefolk's trigger by playing Kha'Zix so he arrives before Faefolk resolves and before Vex is there — Kha'Zix is not Stunned and can only trade for Faefolk while conquering. The opponent must declare this immediately when Faefolk's trigger is on the chain; they cannot wait to see your response first.