Hiya,
Vitamin D looses it's potency with light exposure and/or excess warmth - that is part of the reason why it's sold in opaque bottle and why there are specifications on the package as to what temperatures to store it at. The expiration date is a rough estimate of how long the vitamins will last if stored properly, but essentially the worst form of "properly" according to the directions on the package. In most cases, the D3 will still function as expected for months or years after the expiration date. However, that date is the longest the manufacturer is willing to guarantee the product will last due to legal reasons.
So, does it really go bad? Yes, it does. However, it does not become toxic in any way. If you're feeding a variety of plats in your salad and have proper UVB so your dragon can get D3 in other forms, you could likely continue to use the Calcium+D3 you have with no ill effects. That said it is likely your dragon would get increasingly less and less D3 over this time.
That said, I'd go ahead and get a new package. It's like $11, which is worth the security of mind.
Hope that helps,
-Ellen