Defensive rain is just plain more useful most of the time for its debuffs yeah (most enemies have offensive stats of several hundreds, while finding an enemy that has defensive stats that go above 100 is relatively rare). Fairly certain debuffs in rpgm vx ace work by decrements of 25% so you can at most halve an enemy stat.
Vhala's skills seem pretty busted, but you need to remember there's only so much you can do to boost her MAG stat, she can only use bows and shields, so the lean unbow and her starting archer gloves are already the best you can do for her weapons. The only thing that really jumps the eye in terms of giving her a big MAG boost would be the Savant's cloak, that might make her viable if you want to use her in an AoE damage comp.
Full ATK Vhala is also sort of okay if you equip her with the tormented shield, which also frees her accessory slot for something like a Ring of Strength, triple shot is a pretty reliable damage spell and you still have the rains solely for debuffs, she's not gonna pop off like Varia since triple shot can't crit, but it is a fair ammount of reliable damage and she has double the ammount of HP that Varia has, just sucks that her mana pool is gonna be tiny with the Tormented Shield.