Filled vials are e.g. dropping from bandits or looted from chests, e.g. minor healing potions with 13/15 filling. A empty vial can be filled again with your own potion. Crafting vials can be learned with two primary skills, but it's enough to get the skill to 30 or 40 unless you want to craft vials of size 100 or so. You have to be lucky to find a player who sells self crafted vials because of the massive drop of activity with the alpha test of MO2 and the release end of this year.