Steaming is great to retain vegetables, but the cooking time seems long. Cook the vegetables over a small amount of boiling water then test after 5 minutes with a fork. The fork should penetrate the vegetable but it should still feel crisp. Why not cook only the amount you need for a meal?
If you do have vegetables left over, cover them tightly and refrigerate. Then reheat in a microwave only to serving temperature (140 degrees).
Vitamin loss, especially vitamin C, increases with air, temperature, water and time. Less water (steamer) and a shorter time (5 minutes) are better.