纯 'pure' is what distinguishes this from regular 素食主义 'vegetarianism'. A 素食 vegetarian may still eat eggs and dairy; a 纯素食 vegan eats no animal products at all. A vegan person is 纯素食主义者 or just 纯素者.
culture
Traditional Buddhist vegetarianism in China (素) historically excluded the 'five pungents' (五荤 - garlic, onion, leek, etc.) but did sometimes include dairy. Modern Western veganism mapped onto Chinese as 纯素 'pure 素'.
