Vinegar won't set the dye used in modern cotton fabrics. It could be that the dye was not set, but it is more likely that there is excess dye that didn't get washed out. Lots of water, soaking, and rinsing should get rid of it. If it doesn't, I wouldn't use it - in anything! It will continue to shed dye, which may or may not be absorbed by the fabrics it comes into contact with, and this is not limited to the other fabrics in the quilt. I wouldn't take the chance.