opponent is defending a battlefield with overzealous fan, when i attack using vi peacekkeeper and a harnessed dragon. will he be able to return my vi back to my base before i can stun a unit?

Yes — the opponent can bounce Vi first. Both triggers fire at the same moment (when designations are assigned at the start of combat), and the non-turn player's trigger goes on top of the chain, so Overzealous Fan resolves before Vi's attack trigger.

Sequence: you (turn player) place Vi's "When I attack" trigger on the chain first; the defending player then places Overzealous Fan's "When I defend" trigger on top (Rule 383.3.d.1, FAQ #2251). The Fan resolves first, moving an attacking unit of their choice — they pick Vi — to your base. When Vi's trigger then tries to resolve, she's no longer at the battlefield, so "here" finds nothing and nothing gets stunned (FAQ #10045). Note they must kill the Fan as the cost when they place that trigger, so they can't wait to see your reactions before committing.

One counterplay: if you play Harnessed Dragon to that battlefield, its "When you play me, kill an enemy unit" can kill the Fan before combat starts, which denies the defend trigger entirely. But if the Fan is still alive when combat begins, yes, they can return Vi before she stuns.