Using older frozen vegetables

Absolutely. That is probably exactly the best use for them if they are still perfectly edible (as they would be if kept frozen), but past their prime. The trick is to choose what vegetables to put together. For instance, if I had a bunch of asparagus I would make a stock specifically for cream of asparagus soup. If I had frozen onions I'd add those to that stock, but I wouldn't throw in carrots, peas or bell peppers. I could freeze that stock for as long as it takes for beautiful, fresh, in-season and inexpensive asparagus to come my way. Then I'd have the key ingredients to make fabulous soup.

Some veggies would add great flavor to chicken stock, others would be best in other applications. Can you be more specific about what vegetables you have?

Frozen vegetables for soup stock should be fine. In fact, since freezing the vegetables will make their cell walls burst, you might even get more taste out of them than you would by just cooking them.

It goes without saying that vegetables that doesn't respond to freezing will probably be bad in your soup too.

Good luck!