In response to him choosing, he attempts to bounce the Ætherling.
I'm pretty certain this doesn't work, but i'm curious. When a player is told to sacrifice a creature, is him choosing and sacrificing a creature instantly done? Or is there time for a player to react to these choices?
Once a spell or ability has begun to resolve, no player gets priority to take any other actions until the original spell or ability finishes resolving. There is no chance to activate an ability between choosing which creature you will sacrifice and sacrificing it.
Even if you somehow could get a different ability to fire off during the resolution of a spell (example: tapping Forbidden Orchard for mana to pay part of the cost that Mana Leak asks you to pay to avoid getting a spell countered) that other ability wouldn't actually go onto the stack (much less resolve) until the current thing was done resolving.
Your opponent could of course dodge the sacrifice by exiling both of his Aetherlings in response to Consuming Vapors (assuming he doesn't have other creatures.)
It's not that sacrificing is an instantaneous event, it's that once a spell begins to resolve, players can't take other actions until that spell finishes resolving.
If your opponent indicates that he or she is sacrificing Aetherling A, he or she will have to do so, as it is assumed that he or she is letting the spell resolve. Your opponent would have to exile the Aetherling in response to you casting the spell, in which case he would just have to choose a different creature. The notable event here is that the player doesn't have to choose until resolution, so once a creature has been chosen, your opponent has indicated that he's letting the spell resolve.