I've found lots of examples of outstanding physical performance under a vegan diet, but I've been unable to find examples of bold theoretical breakthroughs being made under a vegan diet (the closest example I could find was Ramanujan, but there were other things about Ramanujan that suggest that there's absolutely no way most of us could live the same life and then end up in the same place), and my own experience has been really discouraging. After about 10 days on a vegan diet, regardless of my energy levels or legible performance metrics, I'll pretty reliably stop being able to, or stop being interested in progressing original ideas.
There are lots of possible exits here: Vegans and inventors were rare until very recent history, we shouldn't expect to have many records of people who were both, we might end up with none, even if there's no relationship between those things. It's also fairly likely that the effect would just be a result of a creatine deficiency, or a choline deficiency, or something like that, which can be fixed with a supplement (although Dr Gregger doesn't recommend it due to the incidence rate of high contamination in creatine supplements, but maybe you can find a brand you trust.)
(Anecdote on choline: I have a vegetarian friend who, at some point, stopped thinking in the sort of focused/precise/fluid/driven way that our project needed. They speculated that it might have been because they were on choline initially, then stopped. So I said yeah, I noticed a change, try getting back onto the choline? I don't think they ever got around to it. The lack of concern that they showed is actually one of the things that bugs me about this, because I also experience that, when I'm low: It kinda seems like we have be on the supplement to hold onto an understanding of why we need to stay on the supplement, and as soon as we lapse we forget what it was like, and how important it was? (Depression also seems to work this way. A depressed person often cannot imagine or remember not being depressed.))
There does seem to be a consistently replicated (edit: creatine findings not consistently replicated, actually!) finding that supplementing creatine enhances memory and intelligence in vegans. (and doesn't for omnivores) (baseline cognitive performance in vegans is similar to omnivores, but personally I'm kind of expecting it to turn out that to be a result of vegan-leaning demographics starting on a higher base, then being lowered)
But even if it can be fixed with a supplement, I don't think most of us are taking creatine and choline! We probably need to have a conversation about that.
Previous discussion of health and veganism, when they touched on concerns about cognitive impacts, generally failed to allay them.
"genius" doesn't seem usefully precise to me? (Is a genius even still a genius once they've found their way into a part of the world where their level of pragmatic creativity is ordinary?)
I'm looking for a sort of... ability to go for extended, rapid, complicated traversals of broad unfamiliar territories in your head, alone, without getting lost, and to find something of demonstrable value that no one has ever seen before. That kind of thing.
That list might be a good start, but I don't know. Can you show us examples of divergent, multi-stage, needle in a haystack breakthroughs that those people made while they were years into a vegan diet? I haven't looked closely at really any of these peoples' work, and there's a kind of relevant reason for that. A lot of them (Singer, MacAskill) are mostly apologists. They work mainly with familiar premises in highly legible ways. The reason most people read them is the reason I don't read them, and the reason I am concerned that a vegan diet tends to limit the ways people can think.
Tomasik is an interesting example though, I've gotten the sense that he has that character, but haven't seen any intense output from him. Recommend some?
Examples of EA-adjacent people who I'd consider to have this quality include Yudkowsky, Wei Dai, Vanessa Kosoy, Robin Hanson (none were vegan during their breakthroughs afaict?).
It might be worth asking which way the causality's running here. A very EA-charitable answer might say something like: "Being humble and accountable (which leads to doing less risky, more legible, and more approachable work) probably raises a person's inclination to become vegan." (It's kind of interesting that, as far as I can tell, long time vegans, the Brahmins, would argue the opposite causality: "Being vegan decreases the mode of passion, but that's good for your spiritual path.")