我是命但不是维他命
[wŏ shì mìng dàn bù shì wéi tā mìng]
This internet name plays with words. "我是命", directly translates to "I am fate/destiny", and "不是维他命" means I am not 'vitamin'. It may symbolize that the person sees themselves as playing a significant role in some aspect of life - perhaps influencing or deciding matters; however, not as fundamental or universally beneficial like Vitamin C etc.