I don't see how being on vita would mean you can't have awesome customization... In fact PSO2 is on vita now as a beta.
Anyway on topic, aesthetically the customization for RO is meh. Gameplay wise the customization comes in the form of cards with different skills and attributes that you put on your clothes, such as attack, critical, improved knockback, improved healing, ect...
The classes play different from each other. LOL Don't know what else to say about them. Some are better for stunning enemies and causing area of effect damage, and some are better for chipping damage and dodging.
The different moves you can do require you to do easy to remember combos. Not everybody is a fan of that but I personally like it since it forces me to set up a move rather than just spamming the strongest move over and over again.
Builds are defined by skills. Your weapon can hold 2 skills and can equip up to 8 cards that can also hold 2 skills.
That's a total of 18 skills to customize your "build" the best build is DS because in a game about farming, you want to do the most amount of damage possible. The Mage and Cleric are the only two classes AFAIK that have viable non - DS builds. They are gimmicks and don't maximize damage.
This hand of mine is coursing with the Power of Magnetism! It tells me to crush Capitalism! Here I go, GADGETO FINGAH!!!