The "Healing Potion", called "Vials of Blood and Gold", were kept in plain glass reliquaries hanging from a leather cord. The formula consisted of the bones and body parts of "supposed incorruptible bodies -- bodies that resisted decay and were worshipped as saints" whose bodies did not decompose, and who were secretly Vampires because they did not age, and were in reality, sleeping. They were, like the original Vampire Guardian Angels, hidden and preserved by the Church and their armies. Some of them were even hidden in plain sight, in glass coffins in plain view of their gullible human worshippers.
In order for the potion to work, it had to be mixed with Vampire blood, and not just any vampire--they had to be ancient "holy ones", since Vampire Guardian Angels were of celestial origins, the still-flowing blood of the incorruptibles, and gold, because of its magical alchemy properties and also because ancient holy relics were kept in gold and became a supernatural preserving agent. What humans did not know was that even the supposed “holy humans” had been bitten by the original Vampire Guardian Angels, and when they "died” or were injured, they were put into a deep sleep, and parts of their bodies were remodeled with wax to make them appear lifelike.
The "blood" was taken from these special "holy relics" and ancient vampires and mixed with liquified or soft flaked gold to form the valuable potion that heals injured Vampire Guardian Angels. This potion was discovered by Prince Jeromos when he was tasked by King Janos to find a cure for the Vampire Guardian Angels affected by the plague, after a gold container filled with vampire blood in which he placed a body part --a finger-- for preservation began to regenerate and move. Later, after Jeromos kidnaps the Author, the vials are mixed with some of Ms. Price's own immortal vampire blood, making the potion even more powerful since she is the "creator" and her blood has full healing and regenerative properties.