I shot a vest that was expired by two years a couple of years ago. It stopped FMJ in 9mm and 45ACP. Then I shot it with a 12 ga slug and 000 Buck. It stopped the buck really well, but I'd avoid getting hit with a slug. It didn't penetrate completely, but it was sent rolling downrange several feet.
The expiration dates exist mainly as a liability preventer; how long should a manufacturer be required to stand behind the product? They make money by selling replacements and by staying out of lawsuits. For liability reasons the vest has to expire SOMETIME.
Replacing the vest every five years is good practice though. Vests keep getting more and more flexible, light, bullet resistant, and comfortable. My first vest, a level 2A was almost 3/8 of an inch thick. Now my level 3A is about a quarter inch and much more comfortable.