I agree the name doesn't sit very well for me either. Perhaps it will grow on me with time.
My problems with it based on my OCD tendencies:

1) Other job names can give some clues about the nature of the job (especially based on final fantasy lore).
2) The new job adds another job that is removed from the "class" system; we may as well get rid of the whole thing if we keep ignoring existing frameworks.
3) The Rogue class is called "双剣士" (Dual Sword User) in Japanese; it seems natural for Viper to progress from this class instead of being a stand-alone.

With a class name like Viper, my immediate image is a whip-like weapon or at least swords / spears with serpent-like blades.
(Such as the "Serpent Blade" used by Zhang Fei in the Romance of the Three Kingdoms or "Dynasty Warriors")

That, or a job that dabbles very deeply in poisons, but that doesn't sound very "Warrior of Light"-ish either.