if you're deficient in any of the vitamins or minerals you will develop clinical symptoms. seems pretty believable to me.
The metabolism of vitamin C in particular is pretty well understood, and no magical benefits of getting huge doses has ever been proven to have any of the claimed positive effects (despite many prominent supporters' efforts, including Mr Pauling).
If you don't eat meat, it will be hard to get iron and B12 and carnitine and some other things. It will also be harder to get enough calories.