There's a reason why there are practically no vegan power athletes. I was vegan for nearly ten years and I regret it immensely as I now see how badly it set me back.
That said, you're right, with a properly-designed diet you can be healthy and happy. I don't believe you can ever be athletically optimal, however (aside from perhaps pure cardio sports).
Aren't supplements part of your diet? Maybe this is a domain term that operates differently. Many people who aren't vegan that I know take a daily multivitamin, does this demonstrate that their diet alone isn't cutting it either?
That said, you're right, with a properly-designed diet you can be healthy and happy. I don't believe you can ever be athletically optimal, however (aside from perhaps pure cardio sports).