Hi Clark,
I wasn't vegan but was a strict vegetarian up to my surgery (15 years ago). I found it very difficult to follow this diet post surgery and ended up becoming an omnivore again as I just couldn't get enough nutrition from plant-based foods given I couldn't eat half of them. Soy was great in the form of tofu, because it was easily digestible, but anything with a lot of fiber (quinoa, couscous, bran, many veggies/fruits,, etc.) just made me feel Ill all the time. In hindsight I'm not sure if it was a lack of protein needed for healing, lack of vitamins, or moreso because it aggregated my pouch and caused watery stools to the point that I was constantly dehydrated and food moving through so fast that I couldn't absorb a lot of it. On top of that i became lactose intolerant post surgery (although vegans would eat this anyway I guess), and all the dietary restrictions just made it really hard to have a balanced diet and control pouch output. I felt a world better when I ate less fibre and more bulky foods (rice, potato, pasta, bananas, applesauce, only cooked veggies, and cheese and for me,, animal proteins, but respect this may not be an option for you). If you can find a plant-based protein powder in combination with cooked fruits and veggies, maybe that might help to slow things down while still getting a lot of nutrition. And if u take supplements, investigate liquid forms--my dr. And I discovered just 5 years ago that pill form multivitamins don't get absorbed for me--who knew?!
Good luck to you.