Does vitamin c have a pro-oxidant effect if taken right after exercise or a workout? should i avoid vitamin c after a workout? how long after?

No, no, and why care: A Vitamin is a molecule needed by biologic systems that can't synthesize it. The amount needed is microscopic. The notion that if some is good more is better sustains a multi-billion dollar supplement industry that ignores conditions caused by hypervitaminosis. As we begin to understand the harm caused by supplemental oxygen, a strong oxidant, our perspective on what's "good" for us is changing.