It's fine to cook your veggies, but you should cook them in as little water as possible or steam them and cook them tender-crisp (not mushy) to preserve as much of the vitamins and nutrients as possible. Adding butter after they are cooked is not only permitted, but encouraged. The only exception that I can think of is tomatoes where the more cooked they are the more antioxidants (licopenes) they release. Happy eating!