Might need a script for that. The only place I could find anything that says "Ignore Defense" is in the Skills tab and it was even hard to find there (basically, the damage formula for skills lets you ignore defense). With a weapon... I'm not sure how you'd do that. I don't think I'd ever tried to make a weapon that "ignores defense".
The closest I have to that involves simply different rates of damage for certain kinds of weapons that fall into the category (like blunt weapons do 200-500% more damage depending on material they're made of against heavily armored enemies).
You can put a skill on a weapon, just make a skill that ignores defense and put it on your weapon.
Though you will have to use the actual skill instead of normal attacks, also it will be a skill unique to that weapon so you don't have to worry about your characters learning the skills if you don't want them to.
Otherwise you will need a script to do that, just google rpg maker vx ace weapon ignoring defense. I've noticed it's easy to find stuff you're looking for if you type in rpg maker vx ace and then search parameter.
without scripts, you cannot since damage is based on the skill's damage formula so normally, you can only modify how damage is calculated on skills. So you would need either a script that will allow you to do just that, or a script that allows you to make damage based on weapon, or a script that allows you to modify the skill used as normal attack for each weapon.
Depends now on where you want the "ignore defense" to work. If you want to make it work for any skill that you use as long as that weapon is equipped, then you cannot use the skill modification scripts
This script allows you to change weapon/guard skills by placing note tags. It works for actor, class, and weapon note tags. It's one of many out there but is a pretty easy one to set up. All you'd need to do is create a duplicate attack skill that doesn't include the b.def in the damage formula and then put the proper tag in the note box of the weapon you want to use it.