Vitamin C is a popular antioxidant and immune system booster. Its antioxidant properties, however, make it unsuitable for ingestion around a workout. Antioxidants can reduce oxidative stress that accumulates during exercise. Oxidative stress may act as a growth signal, so reducing the amount can impair gains. Take your Vitamin C dosage at least 4-hours away from your workout and avoid dosages higher than 500mg/day if your goal is to maximize gains.
Reference: Paulsen, G., et al. (2014). Vitamin C and E supplementation alters protein signalling after a strength training session, but not muscle growth during 10 weeks of training. The Journal of Physiology, 592(24), 5391-5408.