I use vinegar all the time to help with fabric setness. That is what my mother taught me to do although I have heard that it doesn't do much with dyes these days.
I use a quarter-ish of a cup with my front loader and used a half-ish cup with my top loader. I put it in right with my detergent. When using the top loader, I put the water and vinegar in and let it swish a bit before putting the fabric in. With my front loader I just put it in the dispenser. I alway put vinegar in with any red or black load or towel load since it will also cut musty smell of used towels, allowed to sit damp.
I have never found it to set filth, just colour and rid smells. I also only use cold water. The vinegar also (my mother told me) helped to cut down on suds some and make soap easier to rinse out. Great with something like a quilt that is so heavy and dense when wetl but that you really want to get all the soap out of. I always use the extra rinse setting for a quilt and then air dry.