Preventing Diced Vegetables Added to Simmering Liquid From Becoming Mushy
Once you add the vegetables to the broth, they are going to begin absorbing water and becoming softer and eventually become mushy. 

This effect is slowed but not eliminated when the soup is cooled.

The only robust way to prevent it is to only add vegetables to the portion that you are going serve immediately, and then serve the soup promptly when the vegetables are cooked to your liking.