Does yellow Vi effect works if i play her ambush when the showdown already started.

Yes. Vi, Peacekeeper played via Ambush into an ongoing showdown joins that combat as an attacker, which fulfills her "When I attack" trigger (FAQ #10034, #10437).

Sequence: you play her as a [Reaction] to a battlefield where you have units (Ambush, Rule 822). When she enters, she gains the Attacker designation, her trigger goes on the Chain, and on resolution you stun an enemy unit there.

One catch: she must still be at that battlefield when the trigger resolves — "here" is checked at execution (Rule 359.3.f.2). If she's killed or moved in response, the stun fizzles. It also only triggers once, on entry, not for each damage step.