Bullet “resistant” is correct. Bullet “proof” would be a tank. Inside an armored landing craft. Moored at a concrete quay.
Now then, since the invention of Kevlar and Spectra fibers, most vests have worked very well, given their use and what’s known as the “threat level”.
Vests are “rated” for what they’ll stop- Level I, Level II, Level IIa, etc. I don’t know the particular values, but it was- perhaps still is- where a certain level will stop up to a certain size and power round. I believe, though my memory may indeed be faulty, that the “standard” for police and civilian armor was L2a, which would stop most 9mm, 45ACP, low-vel .44 mag, some .357, etc. In other words, most- but not all- typical handgun calibers.
As I said, they’ve worked well for years. The only improvements being in size and comfort: A newer L2a vest is thinner and lighter than, say, a ten-year-old L2a vest.
The vests work by, quite simply, not allowing the threads to tear. If the fabric does not rip or break, the bullet is blunted and does not penetrate. Better (higher threat level) vests both provide more “padding” and also are strong enough to resist non-deforming bullets.
You also have to remember that the bullet is “defeated”, yes, but most of the kinetic energy is still transferred to you, the wearer. A person can easily be significantly injured simply from the blunt trauma- broken ribs are common, massive bruising is a given, and it’s generally an unpleasant event all around.
To answer your question, yes, the newer the vest or armor (they have arm, leg and face guards too) the better it is. Not only from a comfort and weight standpoint, but the materials the vests are made of tends to degrade over time. Years ago, it was a common PD requirement that vests be replaced every three to five years. They also couldn’t be washed in the conventional sense.
Which also means, don’t trust your life to an older, used vest- it may not be a “true” L2a anymore.
And to be perfectly honest, if you are planning to go somewhere or do something where you feel you’ll need armor, by far the best solution would be to NOT go there or do that. Vests do NOT make the wearer invulnerable.