Most everything lasts a while past expiration
before deterioration, including food. Expiration
dates have a margin of safety built in. That being said, the above is true, but vitamins usually have way more that anyone needs in them so a little deterioration in potency isn't that big a deal depending on why you're taking them. After all, much of them just goes down the toilet anyway, so again, a margin of safety is built in there, too.