Dragon Quest IX

[Vampyr]

Hmm, off the top of my head, I think this is all the classes:

Warrior
Martial Artist
Mage
Priest
Thief
Minstrel

Gladiator
Luminary
Sage
Paladin
Ranger
Armamentalist

The second group there has to be unlocked, which I think you do by performing certain quests, not by reaching a certain level in a class. Once a class is unlocked it's unlocked for everyone.

There isn't a huge benefit to staying in one class. There is a lot of overlap in what skills the classes have available, so you can actually farm skill points by levelling a character in one class, then switching back to their "real" class. So my priest for example, I'm currently levelling her as a mage and putting all the mage points into wands. Then when I switch back to priest, I'll be able to level as a priest and use the priest points on the priest vocation skill. (Each class is levelled invidiually, so one character could be a level 50 page and a level 10 priest for example)

Likewise, if you invest 100 points into a weapon skill, you can use that weapon on any class. So if I wanted my martial artist to be able to have a shield I could level him as any class that has access to one (warrior, priest, minstrel, etc) and build up his shield skill to 100.

So really, there are enough classes to where you can farm points to get most things you want, so it's better to switch around.