Vial

A Vial is a consumable item in the Golden Sun series. Like with most consumable items, each party member can hold up to thirty vials in an inventory slot. When used, a vial restores 500 Hit Points to a single party member. Vials can be bought for 500 coins and sold for 375 coins.

Golden Sun Vials
In the original Golden Sun game, a limitless supply of vials can be won from the Lucky Wheels minigame in Tolbi.

Vials are dropped by Goblins, Skull Warriors, Golems, Apes, and Ant Lions. Also, the Living Statues of Altin Peak and the Mimic in Vault Cave are guaranteed to drop vials upon defeat.

The Lost Age Vials
In Golden Sun: The Lost Age, vials can be bought from the item vendors in Loho and Prox.

A limitless supply of vials can be won from the Lucky Wheels minigame in Contigo.

Vials are dropped by Raging Rocks, Rocs, Golems, and Macetails. Also, the King Scorpion and the Aqua Hydra (the bosses of Yampi Desert and Piers' boat, respectively), as well as Briggs, are all guaranteed to drop vials upon defeat.

Dark Dawn Vials
In Golden Sun: Dark Dawn, vials can be bought from the item vendor in Tonfon.

Vials are dropped by Sirens, Faeries, Ravagers, Mad Plants, and Fell Dead.

Analysis
Although they are not so powerful as potions, a limitless supply of vials can be purchased from item vendors, making them common items in players' inventories near the end of the game. The exception is the original Golden Sun, where vials don't appear in stores, giving the position of end-game healing item to nuts. Although vials can be won as monster drops or by playing Lucky Wheels in Tolbi, so could the vastly superior potions, effectively making vials obsolete.

The usefulness of vials depends on the player's combat style. Herbs are invaluable early in the games, while Djinn are rare and Psynergy Points are a valuable commodity. But by the time vials come available, players will have an abundance of PP for healing spells like Cure and Wish, and will probably have acquired some healing Djinn like Breath or Flower. Therefore, vials are more valuable to players who prefer using Psynergy for offense and keep Djinn set to improve their classes. On the other hand, players who unleash Djinn often or who conserve Psynergy for healing may find vials (and other healing items) a waste of inventory space and would be better off spending money on armor. But even these players may find having vials on hand useful if their designated healer is taken down or Psynergy sealed.