When the trigger resolves, Phantasmal Image returns to the battlefield with a +1/+1 counter
Kaalia of the Vast can cheat it from your hand (since it's a dragon), Yuna, Hope of Spira can bring it back from your graveyard (since it's an enchantment), and Esper Terra can copy it and trigger most of Bahamut's value by advancing three chapters: Summon: Bahamut isnt legendary, so Esper Terra has no problem copying it
Once a player has announced that they are casting Cavern-Hoard Dragon, no player may take actions to try and change the greatest number of artifacts an opponent controls before Cavern-Hoard Dragon's cost is locked in
The whole point of Vapor Snag is to just undo the work youve done, even at the cost of a card
However, getting to dig for a hasty attacker as part of doing so, increasing the potency of your attack