what can I do with vegies used to make stock?

You could add them to soups or veggie burgers for extra bulk, but they won't contribute any flavor.

Unfortunately, usually, you need to just toss them (or compost). After sitting in liquid for so long the veggies don't have much nutrition left and are quite soft/mushy. As Julia Child said, "those vegetables have given their all". You might be able to puree them and turn it into soup, but it probably wouldn't be terribly good.