Have you tried hanging spinach leaves from the top or side of the cage? That is one of the only ways I could get mine to eat veggies at first. Since it's leaves and it's in their nature to eat leaves, they went for it first. Now they eat quite a few veggies. You have to just keep trying. (just like with real children!!
Another way I got veggies in them was with baby food. If you are interested in that, I can give you the link to it.
I don't know about vitamins. I know everyone here says no to vitamins in the water because it can cause bacteria to grow.