While not vegan (or vegetarian these days), Clarence Bass (
www.cbass.com) eats pretty close to what Dr. McDougall recommends.
Breakfast is grains with red or kidney beans, frozen berries stirred together. (I have his video and he does add 1 tspn of creatine monohydrate and two tablespoons of protein powder.)
Lunch is whole grain bread with a nut butter spread or Boca Burger, apple.
Dinner is either rice & beans or large salad with a grain side. I believe he started adding <3 oz of animal protein to his evening meals.
While Bass admits to having used steroids when he was 40 for a bodybuilding contest, he notes he has not used them since.
But, looking at his success with a diet that is +60 - 70% starch based, you can see not only is getting a flat stomach possible but getting "ripped" is probable.
BTW, Andreas Cahling was rumored to have used steroids.
And avoid--if you want to your back to stay healthy and strong--any and all "ab machines". Two family members have herniated discs because of those devices. One of the foremost back experts in the world, Dr. Stuart McGill, notes ab machines takes your back out of its normal safety range and compromises your support structure. Crunches on the floor--you only need to move ~ 1 - 2"--is all that is needed.