I think there is a different approach for thinking this through.

Body armor is not quite what many think. There are levels of body armor, and what most don't like wearing with body armor are the extra plates that will stop rifle rounds, such as 7.62 NATO. However, firing stuff like that in a house is dangerous business. It's difficult to get handguns that can fire ammunition to energy levels that will defeat such plates, but who, even criminals, are really going to wear them?

I have a hard time believing that a 3" express magnum 0 or 00 buckshot will not disable someone with even IIIa body armor. The arms and face and lower extremities that are struck with such projectiles can be enough as evidence by shrapnel wounds for military that wear such devices.

When I was in Iraq, the first week we were there a company commander was in a direct IED strike. Essentially, his torso was intact, but the shock to his system on his extremities disabled him immediately and he died from shock. Body armor like that is designed to allow you to survive an attack, not necessarily to be able continue operations. That is provided there is immediate response available and other fighting forces in the area. Shrapnel from even 60mm mortars took down people wearing full body armor; I knew a soldier (wearing full body armor) that took a sliver of 60mm mortar that struck right in the throat and he barely survived; he was immediately incapacitated.

Were it me facing the threat you describe, I would load up a 3" magnum 00BK, maybe an autoloader for a threat like that. With that, you really don't have to aim, and only one pellet needs strike critical area. Handguns are chancy devices against a well armed and armored enemy, and rifles are easy to aim incorrectly, unless you are extremely well trained. At very close range, the shotgun always rules, and doesn't require nearly as much training or skill to use.

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