If player A play a rengar with ambush ability and player B play tideturner making vex move to battlefield does the rengar stun?

Yes — Rengar gets stunned, assuming Vex was at a battlefield when Rengar entered.

Vex's ability is "When an opponent plays a unit while I'm at a battlefield, [Stun] it" — the only thing that matters is Vex's location at the instant the unit is played (Rule 383.2.c). When you play Rengar via Ambush in reaction to Tideturner, Tideturner's swap trigger hasn't resolved yet, so Vex is still where she started; if that's a battlefield, her trigger fires, stuns Rengar, and locks him from moving this turn. Ambush doesn't bypass it, and it doesn't matter that Rengar was played elsewhere (FAQs #10318, #11632).

The one exception: if Vex was sitting at a Base when Rengar entered, the trigger doesn't fire — and Vex arriving later via the swap won't retroactively stun him.