Before completing Lunar Diplomacy, collecting empty vials allows for affordable, but time consuming, Herblore training. After giving the lunar equipment set and waking sleep vial to the Oneiromancer, and getting back with magic kindling, use the potion on the kindling, drop the kindling and ask for another potion and kindling. The resulting empty vials can only be filled with water during the quest, however, Guam and Marrentill can still be added to filled vials of water after the quest has been completed.