Generally what makes vests degrade is exposure to heat, moisture and UV light, i.e., sunlight. If a vest is properly stored the company *guarantees* it's good until the expiration date. After that, the company assumes no liability if the vest doesn't work as advertised. However, it may still provide a lower level of protection for years and is certainly better than nothing.
I have purchased old kevlar panels off of eBay and used them for ballistic testing. Keep in mind that it isn't just moisture and the like, but body sweat.
Now, I don't know that any of the panels I purchased were abused or not. I don't know if they were fully capable of their original rating. I had panels from the late 80s to late 90s and none failed with typical ball .32, .380, 9mm, or .45 acp and never with hollowpoint (which is more easily stopped by ballistic vests than ball because of the hollowpoint's rapid expansion allowing it to be more easily 'caught' by the fibers). All of these were calibers within the ratings of the panels. I even had a panel about 6 years out of day (11 years old) that we shot several times with a .44 magnum at about 5 yards. I will be honest, some of the panels I tested were just plain nasty with body odor and sweat stains. They still worked quite well.
Note, I worked with almost exclusively IIIa vests, but occasionally a II vest. My tests were for insight into how they work. We also shot a couple with calibers outside of their ratings, like 5.56. With the first few shots at 25 yards, I thought something was wrong with my sights as I must have missed the panel since I never saw it move. I had my buddy spot for me and with a spotting scope, he assured me I was hitting the panel as he could see the cover 'flinch' slightly. Sure enough, there was about a dozen little holes in the panel and not a single round stopped.
Maybe I mistyped. I did have panels fail, but ONLY after having suffered massive damage or when a round passed through a hole already created by another round. NIJ used to rate panels for 3 shots. I had panels into which I put 30 or more 9mm and .45 acp before becoming so damaged they started failing. Kevlar is good stuff.
Bottom line, I would be estatic to buy a new, slightly older kevlar vest for $70. I would not doubt it in the least for the calibers for which it is rated.