I was so happy that using Oxyclean, vinegar, and time outside bleaching in the sun worked beautifully! It was really kind of miraculous, actually.
Here is what I did once I bought Oxyclean White Revive:
1. Put a large rubbermaid tote into the bathtub, added a scoop of oxyclean, and filled the tote with the hottest water I could get from the tap. Pushed the duvet cover into the water and let it soak overnight.
2. Washed the duvet with vinegar in the washing machine
3. Hung the duvet out to dry early in the morning and let it sit in the hot sun all day.
4. Repeated steps 1, 2, and 3 two more times
This project really wasn't much work, though it took a few days to finish. That being said, I just saved myself a lot of money by fixing a beautiful duvet that I really didn't want to get rid of yet (and honestly couldn't find a good enough replacement for). In addition, I am planning on doing this with all of our white linens and clothing every so often to keep them beautiful... maybe I'll even get really wild and start buying white towels!