The science about the health benefits of vitamin C is, to say the least, fraught with controversy, dating back to when Nobel laureate Linus Pauling first suggested it might help fight the common cold and cancer. Two sides of the issue: from Alacer, makers of Emergen-C, versus Stephen Barrett's "Quackwatch" .
My bias: I use Emergen-C as part of my supplement regiment - if nothing else it's got B-12 which is good for vegans to supplement. (I have confirmed with Alacer that it is vegan, except for the products with Glucosamine and Chondroitin.) And I generally find "Quackwatch" to be a self-righteous and close-minded defender of current medical orthodoxy - but he gives references, so check it out yourself. Read the research and make up your own mind - it's your body.