I drank Emergen-C all through my pregnancy, probably 2-3 packets per day. It was the only thing that helped me drink as much water as I needed. My midwife never said anything about it other than that she carried it in her birth kit.
After all that consumption, I had a healthy 9lb 2oz baby at home, 16 days after his due date. From what I've read, vitamin C strengthens the amniotic sac, so I wonder if that might be why he came so late. My water didn't break until right before he was born.
Excess vitamin C is excreted in your urine, so it does not build up in your body - it's a water-soluble vitamin. I personally wouldn't be concerned about it and if there's another pregnancy in my future, I will probably drink Emergen-C through that pregnancy, too.