Despite having no elemental affiliation, Vidatu has the benefit of constant NulAll and cannot be hit with any magic except Drain, Ultima and Holy. He has Auto Regen. Vidatu's attacks will mainly involve Ultima, and Osmose, which can take a large chunk of MP from a character with the highest current MP.
The best option is to use the Celestial Weapons at full power (if available) for defeating him.
Vidatu is not immune to Magic/Mental Break, however, it will only work 1% and 20% of the time, respectively, because to its Magic/Mental Break Resist.
While Vidatu is not immune to Armor Break, it will only work 20% of the time. A Frag Grenade or Auron's Banishing Blade Overdrive work 100% of the time, and so are more effective to break its Full Break resistance.
Asto Vidatu is a death deity in Persian mythology.