Veggies shouldn't be instead of anything else in a bulk diet - they should be in addition to other foods. They provide the vitamins, minerals, antioxidants and other biochemicals that are fundamental to enhancing health.
So what veggies are good for bulking then? ALL of them! Get a good variety, have some at just about every meal (yes, all 6 or 7 or 8 of them) and change it up on a regular basis.
A good book for the basics of which veggies contain which beneficial substances is "The 150 Healthiest Foods" by Jonny Bowden. Good stuff, a standard reference in my house.