Alchemists Vial rules
You are able to activate it anytime you could cast an instant.

However once a creature has been declared an attacking or blocking creature the Vial's ability doesn't do anything, since it prevents a creature from being declared an attacker/blocker but doesn't undo that declaration.

From the Gatherer Rulings:


  Activating the last ability of Alchemist’s Vial targeting a creature that’s already attacking or blocking won’t cause that creature to stop attacking or blocking. It will prevent that creature from attacking or blocking in any additional combat phases the turn may have (although this is unusual).