I did it for six months. By the end of the six months, I felt like a doormat. Seriously low energy. Blood tests showed better cholesterol, but low testosterone, low Vitamin D (I live in the desert, so this was weird.) I couldn't function anymore. I was just a doormat.
Energy-wise, I run best on a full keto diet. I have nice even energy and enough of it, and my mood is buoyant (with unpleasant digestive side-effects, however).
On the vegetable-only diet, I felt slow, depressed, and fading eating just plants. I actually wound up taking sertraline because I was cranky all the time.
When I eventually caved and tried eating meat, I felt a lot better. I know that people have told this story before and even telling it annoys vegans, but after six months I was past the "your body will adjust phase." I was lacking something in the diet.
But this was, I am thinking now, an error on my part. I do not process carbohydrates well. I think I may have been taking in an unreasonably low amount; I rarely ate beans, lentils, or grains. Most of what I was eating consisted of stir fries and salads, and vegetable soup -- by themselves, just not doing the job.
Ordinarily, carbohydrates cause me the standard spike-crash problem so many people complain about, but I wonder if that made me over-paranoid about them.
I'm easing into a new diet now which will in a week or so become plants only, and what I'm doing is eating a little grains, legumes, but this time with every meal, portioned out to a measured single serving (and all high fiber, low-glycemic index carbohydrates).
No more greens-only meals (which I'm fortunate enough, I guess, to actually like). I am hoping the effect will be that I can handle small amounts of carbs eaten regularly throughout the day, and that they will take care of whatever that low energy thing was I got the last time I tried.
What I did like about the vegan diet is this odd feeling of all my joints loosening up, like when they gave the tin man oil in The Wizard of Oz. I enjoyed moving around in that sense; I just didn't have any energy to do it.
I'm well-stocked now with quinoa, lentils, bulghur wheat, and some very high-fiber noodles. We'll see where that goes.