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

No — Rengar will be stunned. You can legally make the play (reacting to your own Faefolk trigger with Rengar via Ambush is fine — that's a valid Reaction window), but Vex's stun fires anyway.

The reason: when you play Rengar, Faefolk's trigger hasn't resolved yet, so Vex is still at its current battlefield. Vex's ability only requires that she be "at a battlefield" when an opponent plays a unit — it doesn't care where the unit is played or how (Ambush included). Per FAQ #9240, any enemy unit played anywhere on the board gets stunned while Vex is at a battlefield. Chain order doesn't save you: Vex's trigger goes on top of Faefolk's, resolves first, stuns Rengar (and locks him from moving this turn), and then Faefolk pulls Vex over.

One caveat: if Vex happens to be at a Base (not a battlefield) when Rengar enters, the stun won't trigger — but in the typical setup where Vex is at a battlefield, Rengar comes in stunned and contributes no Might in the ensuing combat (Rule 423.1.b).