Can i react to my own faefolk trigger targetting vex apathatic ambush rengar in without getting stunned?

Yes — play Rengar as a Reaction to your own Faefolk trigger (before it resolves), and Vex will not stun him.

Vex, Apathetic's ability only fires when an opponent plays a unit while Vex is at that battlefield. Faefolk's trigger is still on the Chain when you play Rengar, so Vex is still at her old location — Rengar enters before she arrives, the trigger condition is never met, and no stun happens (Rule 383.2.c). This is the same window FAQ #11573 describes: your own Faefolk trigger puts you in a Closed State with priority, and you can jump in before the pulled unit lands.

Note the contrast with FAQ #10932/#9735: those Ambush units were played after the pull resolved, with Vex already on the battlefield — that's when she stuns. Also check your Rengar's permissions: Rengar, Pouncing can come in via "a battlefield you're attacking" (your Faefolk move made you the attacker), but Rengar, Trophy Hunter's Ambush needs enemy units already at that battlefield — if it's empty, he can't legally be played there at all.