Rabbits are herbivores, not vegetarians. Weeds such as dandelion, white or red clover, plantain, chicory, raspberry/blackberry/strawberry leaves, prickly lettuce and sow thistles are far superior for rabbits than garden vegetables. See the Safe Plants list in the Natural Feeding for Rabbits forum for ideas and botanical names.
Garden vegetables are less desirable. Many like carrots, beets and corn contain more sugar than is healthy on a regular basis. Other, such as cabbage, broccoli, Brussel sprouts, kale etc. can cause intestinal gas, which can be a problem. My rabbits have adjusted and can handle just about anything I throw at them, but they have several generations of natural feeding behind them and even so I am careful when it comes to fruits and garden vegetables.