If I use a Vi to attack a Glasc Mixologist and after it dies, it summons a unit to that battlefield. Does my Vi get to stun that unit again?

No — Vi only stuns once per attack.

Vi, Peacekeeper's ability is "When I attack, [Stun] an enemy unit here." That's a single attack trigger (Rule 383.4.e) — it fires exactly once, when she first gains the Attacker designation, and the stun instruction is fully carried out on resolution. There's no Repeat or any "if it dies, repeat this" text, so nothing re-triggers off the Glasc Mixologist dying. This matches the established ruling for Vi, Peacekeeper (FAQ #10205).

Timing makes it doubly moot: the stun resolves during the Combat Showdown Step, while Glasc Mixologist's Deathknell unit only enters during/after Combat Cleanup, well after Vi's trigger has finished resolving — and that new unit will be ready, unstunned.