I have a juvenile beardie as well about 5-6 months old, and I have never seen him eat veggies. I offer them regularly and I've tried several types of different greens. Spinach leaves, kale, bok choi, carrots, mustard greens, etc.. with no results.
However I read for young beardies that the ratio is about 20-80 with eating veggies vs. live insects
Youtube has many helpful videos. Some recommend placing larger leaves for him to nibble off of, some people take a long probe and move the greens around while he is in front of them to make them appear to be alive. Some people recommend dusting the greens with calcium powder. But I have had no luck
As far as his bowel movements I had him on a steady diet of crickets and he was pooping fine. However I switched to larger mealworms and he had a hard time pooping.
A trick I learned was to place him in warm water under his neck, and not too much where his legs are floating. Not too warm though! And every time he would let out a huge long poop. Im sure like the other users posted the higher hot spot temps will speed up digestion but this worked for me every time.
As far as boosting other vitamins from veggies make sure your insects are gut loaded meaning they are eating veggies you want to feed your dragon. I feed my crickets the standard carrot feed from petco or petsmart. I also feed my crickets Nature Zone Bites which are actually for dragons but the crickets love em.
Hope this helps