Keeping in mind that I haven't looked at RMXP for a long time, I think your problem is that you are only running the appropriate part of the method when equipping a weapon:
if equip_type == 0
If equip_type is 0, it means you are equipping a weapon. If you changed that to the appropriate type (by memory: 0 => Weapon, 1 => Shield, 2 => Head, 3 => Body, 4 => Accessory), then it would run when you want it to. Judging by your naming, you want it to be body armor, so try doing it with
if equip_type == 3