If vi attacks and it stuns a unit can i flash to not stun my unit

Yes. Flash your unit to base in response to Vi's trigger, and the stun won't apply.

Vi's ability reads "When I attack, [Stun] an enemy unit here" — "here" is checked when the trigger resolves (Rule 359.3.f.2). Once your unit is at base instead of the battlefield, it's no longer a legal target, so the stun instruction is ignored (Rule 359.3.e.5). This is the same principle as the established FAQ rulings where moving Vi's stun target away with Gust makes the stun whiff.

Note the timing: you must play Flash (a [Reaction]) while Vi's trigger is on the Chain, before it resolves.