When does Vision trigger on a unit that gains it from Gemcraft Seer, specifically when revealing a hidden unit?

Ruling: Vision triggers immediately when the unit is played (resolves). The unit is on the battlefield when it resolves and can "see" any play effects it has, including those granted by passive effects like Gemcraft Seer.

Sequence:

• The hidden unit is revealed and played
• The unit resolves as a permanent and enters the battlefield
• Vision trigger is immediately added to the chain
• Players receive priority to respond to the Vision trigger
• The Vision trigger does not wait for the entire chain to resolve before triggering

Nuances:

• A unit can trigger play effects granted to it by other cards (like Gemcraft Seer) even though it doesn't inherently have those abilities
• "Resolve" and "played" are functionally equivalent in this game for all intents and purposes