Personally, for the purposes of lotting, I would consider it an accessory, but I can see arguments either way.
Arguments for armor:
• Provides defense
• Left side of character pane
• Provides VIT for all classes
Argument for accessory:
• Excluding weapon+offhand, there are 6 slots on the left, and 5 on the right. However, since two of the right slots are rings, calling the belt "armor" would make it 6 armor drops and 4 accessory drops. From a "who gets drops" perspective, in order to make things fair and balanced, it makes more sense to make the belt an accessory so both people get a pool of 5 drops to lot on.