Vegetable Spell is a quest item in Divinity II: Flames of Vengeance. It can be obtained during the quest Seahorse Salad if you choose to battle Ursula in the Hellish Dimension instead of letting her go. When defeated she drops the key to a chest which contains the spell.
The spell can be used anywhere at any time to return Gardner, Ashraf, and Thomas to their human form. They can be found in the Prancing Seahorse afterwards, and offer the Main Room Chest Key, Medicine Chest Key, and a Skill Book as rewards.