India seems like the obvious answer. And estimated 30-40% of the population practices vegetarianism. You could probably boost that number, and there are certainly points in India's history where you could turn it into a superpower. It is in the nuclear club ...
But that seems like too much work.
Allow me to go for an easier but probably less likely idea. I'm not exactly an expert on Buddhism, but I believe some forms of it require monks to be vegetarians. I believe also that some practicing Buddhists embrace vegetarianism and consider it a logical extension of their beliefs. If there was a Buddhist sect that encouraged vegetarianism that became the most popular, or if all sects encouraged vegetarianism more, then it's possible a country like China, Japan, or Korea with a high number of self-identified Buddhist adherents might easily become a superpower with a high percentage of vegetarians in its population.
Of course, the same could be done with Christianity or Islam. Somehow, though I find Buddhism a more likely candidate.