I had a Mythweaver Poc in play and an untapped Promising Vein. Opponent played Imprisoned in the Moon targeting my Commander (Bristly Bill) but the game did not give me a chance to respond on the stack and ***** my Promising Vein before the spell hit Bill.

There wasn't anything in play that prevented responses as far as I can tell, so I believe I should have had a chance to respond on the stack and activate my Promising Vein before the enchantment hit my Bill.