My angels don't eat veggies per say,but I include spirulina in their diet.
Now,if they don't get fed,they will attack any plant in my tank,and its not a pretty sight.
Angelfish are omnivorous. Spirulina flakes are a great way to introduce roughage into the diet. You can also clip leaf lettuce or spinach (not iceberg lettuce) or zucchini or cucumber in the tank and let them pick at it - younger ones may need it lightly blanched first.