This is a great site. Instructions I use to clean up my antique & vintage linens.
http://www.antique-linens.com/laundryTips.html

Lisa Capen (Lisa Capen Quilts) often washes yellowed feed sacks then lays them out to dry on the grass as she says the chlorophyll in the grass provides natural whitening.

Yes, Cynthia mentions that too. Plus the sun whitens. I do my older white fabric items this way.
I tried soaking in bathtub, but didn't care for the process. The dirty water is difficult to drain well and remove as much as possible from the fabric you're cleaning.
I get one of my big plastic bins to place in tub. Makes rinsing with fresh, clean water much easier. Squeeze like a milkmaid then lay out on grass.

I have used Oxy Clean on several vintage pieces and have had great results. I put the quilt in a laundry basket with open spaces,
prepared the oxy bath in the bathtub, left it in there for a day, but swished the basket around every couple of hours. I was pleased with the product on other problems, too.