There are a lot of micronutrients that vegetables contain, that would be difficult for a vitamin to contain. Plus vitamins are boring when you can eat real food.
Veggies are not a "supplement" they should actually be the basis of our daily diet. Instead of eating lots of meat with vitamins replacing veggies, we should eat lots of veggies with vitamins replacing meat (if we attempt to replace anything with vitamins.)
No, the "roughage" assists your digestion in slowing the process through your stomach and 'insides'. Also, vegetables have things that you need and cannot get elsewhere. Just think, how long has it been since we (man) has been eating ... food? Answer: a long time. Vegetables are only one of the many things people need.