Vocations are the job classes of Dragon Quest VII and won’t be unlocked until you complete the scenario in Alltrades Abbey in the past. Upon doing so, you can talk to the main person at the abbey to change any of your character’s vocation. They are split into two main types, Human Vocations and Monster Vocations. Monster Vocations are a bit more complicated and will be talked about in their respective section. All vocations follow the same rules, though, in that they level up as you win fights with monsters.
The only catch is that there are unseen level caps throughout the game’s world and if your character’s level is equal to or exceeds that cap, then they won’t gain any vocation experience. For example, the Western Cave in the Alltrades Abbey area in the past has a cap of level 20, meaning that you could grind there for vocation experience if your characters are under that level cap. There are many locations in the game where the vocation cap is level 99, so you don’t need to worry too much about the ones that actually have a cap.
As already mentioned, every Vocation in the game levels up by fighting and winning battles (within the level cap just talked about). Each one has a max level of 8, which is represented by stars, so if you have a Vocation that has eight stars (they will be yellow in the Attributes menu), then that one is mastered. Vocations have no bearing on stats gained from leveling up or from Seeds, so the only stats affected are those reflected whenever you switch to that particular Vocation.
Warning: Spells/Abilities learned from Beginner Human Classes will stay with the character, no matter which one you switch to. However, Intermediate and Advanced Human Classes have their spells/abilities affixed to them, meaning once you change to another class, then you lose access to them