If opponent play rengar to battlefield b and in response i play tideturner from hidden switching with vex to battlefield does the rengar stunned?

It depends on where Vex was when Rengar was played — the swap doesn't change the outcome either way.

Vex's trigger is checked at the moment the opponent plays a unit, based on Vex's location then (Rule 359.3.f.3). If Vex was already at a battlefield (any battlefield — her stun is board-wide, not location-restricted), her trigger fired the instant Rengar was played (FAQ #9240), so Rengar gets stunned and can't move this turn; the Tideturner swap is irrelevant. If Vex was at Base (not at a battlefield) when Rengar entered, her trigger condition wasn't met, so no trigger fires — and swapping her to battlefield B afterwards doesn't create a new "plays a unit" event, so Rengar is not stunned.

Note: Tideturner being played doesn't trigger Vex either (he's played by Vex's controller), and since Rengar's play completed before your response, the only thing that matters is Vex's state at the moment Rengar hit the board.