The Vow is a binding pact between humans and Pokémon created sometime in Pokéarth's prehistory. It is what allows trainers to train mons, and (for the most part) keeps humans off the menu.
While the exact origins of The Vow are uncertain, they are known to be tied to the legends and myths found in the Canalave Library. The myths essentially speak of Pokémon having a purpose and living in harmony with humans; Man takes a Weapon and goes slaughtering Pokémon with it for no particular purpose, until he eventually finds himself alone. Then a mysterious figure appears (out of the void?) and chastises Man for having deprived himself of the presence of Pokémon by his side. Man, distraught, forsakes the Weapon, and with it the mysterious Pokémon figure disappears into whence it came. It also talks of how Pokémon have committed to appear before humans in the tall grass of their own accord.
For almost as long as the Vow has existed it has been represented by a symbol - two halves of a circle, joined together by a straight line and another circle in a center, representing two disparate sets of beings agreeing to work with rather than against each other. This symbol was later used as the base of the design for modern Pokeballs.
The terms of the Vow are what prevented the Seven Jerk Dragons from rising against humanity after the events of the Scolemis War in AU. They find a way to loophole abuse by turning most of mankind into Pokémon. Before them, an ancient Alakagross Civilization had been severely punished for endangering many species, among them the early humans.
It is believed that the Armbands, in particular the Human→Pokémon ones, were created as ways to explore The Vow.
These are what counts as a fair capture under the rules of the Vow:
- The mon agrees to go with a trainer via negotiation.
- The mon challenges the trainer to ritual combat in full expectation of possibly being caught and does. (Ex. The typical "A wild Pokémon appeared!" type scenario.)
- The mon was trying to attack or steal from the trainer directly and gets caught in the process, as those are taken as fair challenges.
If a human wearing a human to Pokémon Armband is captured in a Poké Ball, the Armband mode locks them into their Pokémon form due to being instruments of the Vow, even if they take off the armband. However, Pokémon to human armbands can still work on them. The mode lock effect can be broken by the humon being released, or by an agreement not to be forced into a Vow-based relationship between the wearer and the trainer catching them, spoken or unspoken. Weremons don't have this restriction due to their transformations being independent of the Vow.