Minor Alchemical Vials: Bridge of Despair - Infernal Armouries
Lesser Alchemical Vials: Bridge of Despair - Infernal Armouries - Alchemical Quarters
Common Alchemical Vials: Bridge of Despair - Infernal Armouries - Alchemical Quarters
Greater Alchemical Vials: Infernal Armouries - Alchemical Quarters - Summoning Chambers
Grand Alchemical Vials: Alchemical Quarters - Summoning Chambers - Forbidden Halls
Floors where the items should drop most commonly are in bold.
The drop rate seems pretty even through out from what I've noticed... Also either you're remembering wrong or the quest description is wrong because Grands only drop on summoning chambers and forbidden halls. Either way I think the current way it's set up is pretty flawed since I know numerous people who are trying to trade grands for other vials. The fact they're the only vials that drop in halls and it takes forever to level there means they become more common... The "rarest" drop should not be the most common. No body had extras of other vials because of drop distribution among floors and how easy/fast it is to level through but there are extra grands.