Does vitamin c flush out THC?

No it won't help. The best way to clear marijuana from your system is to quit. I don't know if it does but think of this:

Marijuana makes you more alkaline than normal. So you need to add something acidic. Like salad with oil and vinegar, or dill pickles, or add some vinegar to your bathwater and soak. I would think Ascorbic Acid (vitamin C) would work also. Give those things a try.