I go on a. Open bf with yellow vi, my opponent play rengar on it, can I stun it?

Yes — she still stuns Rengar. Moving into an empty battlefield only starts a Non-Combat Showdown, so Vi doesn't get the Attacker designation yet (FAQ #8664). But when your opponent plays their unit there, the showdown converts into a Combat Showdown, and the Attacker is the player whose unit applied Contested — you (Rule 464.2.c.1). Vi is present at the battlefield, so she gains the Attacker designation at that moment and her "When I attack, [Stun] an enemy unit here" trigger goes on the chain, stunning Rengar.

One timing caveat: if the first showdown had already fully resolved and you'd established control of the battlefield before Rengar arrived, then Rengar applies a fresh Contested, making him the attacker and Vi the defender — in that case no stun (FAQ #7053). Also, as always with Vi, she has to still be at the battlefield when the trigger resolves.