Slugs against armor do less than one expects.
Penetrating less than a .22 magnum by far.

More similar in effect to hitting someone with a ballbat than a penetrating wound.

Some 20+ years ago a LEO took a shoulder hit off hit concealable armor, the thinnest point. The slug actually slid off the armor injuring the officer above the armor. The armor itself stopped the slug from penetrating.

We had a pile of armor supplied by a local police supply shop, we were helping them dispose of them. Slugs were the easiest to stop, maybe not the most comfortable but the easiest.
One conventional class 2 vest took 50 .357Mag hits from point blank range on each panel without failure. A matching vest took 7 rounds in the exact same point before penetration occured. Both vests are still on display last I knew.

The hardest to stop were the high velocity rounds.
No not 9mm but 9×25 Dillon.
One particular 10mm handload showed promise. Need to look at that new Glock model 40 10mm soon as I can.

Rifles made swiss cheese of most concealable armor, if the chicken plate wasn’t hit. This is rifles not pistol caliber carbines mind you.
Until we start talking external armor you don’t even need specialty ammo, factory ball and most good HP or SP loads still make a mess of them. Especially in larger calibers. .308 and .30-06 tend to chew armor up.

Very simple, you put one or two rifle rounds into your target with no effect or you see sparks off the chicken plate, aim where the armor ain’t.
Look at bad guy #2 in the bank of america shootout, how did they take him out? One SWAT guy stuck his rattlegun under the car and chewed his feet out from under him. He bled out there.