Nutriberries are approximately equal though to what they are getting from pellets. However if he is not eating the pellets they may be worth a shot if he likes them. I also give them to Rowdy and Merlin, 1-2 each per day because they enjoy them. Pete has not wanted to try them yet, which surprised me being a seed junkie.
Keep trying the veggies every night. At first Pete was only eating the corn. Now he eats pretty much all of them. It's actually funny to watch him suck up a carrot or green bean, lol! The whole thing hangs from his beak and he sucks it up bit by bit like a strand of spaghetti.
But, yes, I think I would only supplement if the vet wants you to. I know too much of certain vitamins can be harmful to us, so I'm sure that's true for animals as well. I had a student taking extra B vitamins thinking they would make him grow taller. They perforated his intestine and he became critically ill. Thankfully he was ok after a few days in the hospital.