Serpent belt

The serpent belt is a magical belt that grants immunity to the poisoned condition and can be transformed into a poisonous snake. Such a belt was given to Vax'ildan by Thorn. Vax eventually named it Simon, and later entrusted it to Kynan Leore to keep the snake safe. Kynan cared for Simon for a year before the belt was returned to Vax. Vax gifted the belt to his half-sister Velora Vessar during her resurrection ritual.

Properties
The belt grants its wearer immunity to the poisoned condition. It can also be turned into a poisonous snake as a bonus action, using the stat block for that creature included in the Player's Handbook. This can be done once per day and lasts for one hour, until the snake is reduced to 0 hit points, or unless the snake moves more than 100 feet away from its owner. The snake follows verbal commands issued by the one who animated it. While a belt, the snake does not require food or water and is unaware of the time passing.

Kith & Kin
The belt is first seen in the possession of Thorn, who received it as a gift from his master when he was an apprentice. During their confrontation with the Shadewatch, Vax used the belt in its snake form as a distraction against Culwen, and it remained with him after the fight. When Thorn and Vax parted ways, Vax tried to return the belt to him, but Thorn told him to keep it.

Campaign 1: Vox Machina
Vax was fond of the belt and named its snake form Simon, after the young boy Simon Brist. The belt was first mentioned in, when Vax used it as part of a surprise attack on a pair of ogres.

In, Vax used the belt to distract the guards outside the Briarwoods' guest room in Emon. As he was caught immediately afterward, he did not have a chance to pick it up again. Vax later asked the palace guards if they had found the belt, but he was not able to relocate Simon. In, Vex'ahlia presented the belt to Vax as a gift, having made a secret trip to get it back from the guards.

When the party were facing the prospect of a difficult fight against Thordak, Vax entrusted the belt to his protégé, Kynan Leore, for safekeeping. Kynan kept the belt for over a year before Vax reclaimed it. The belt was ultimately gifted to Vax's half-sister, Velora, during her resurrection ritual.

Behind the scenes
Simon was acquired during their pre-stream home game as a random treasure drop of a Pathfinder common item. Matt thinks like the best magical items, Simon wasn't just useful for gameplay, but also became part of Vax's identity.

Trivia

 * Liam O'Brien's character in Campaign 3, Orym, invited Fearne Calloway in snake form to wrap around him like a belt, prompting Matthew Mercer to mention Simon.