Test subjects: Two grungy, embroidered pillow cases found at my dad's house while cleaning it out.
Left side, what they both looked like before soaking. Right side, after soaking one of them for 48 hours. It probably would've taken less time, but I forgot about the poor thing for over a day.
I have also used VTS on my grandma & great-grandma's Sunbonnet Sue blocks that I put into two quilts for my cousins. When I found the blocks, they were nasty to the touch and grayish yellow. After soaking them for 24 hours, they were much brighter. They very nearly matched the brightness of the muslin I used as a background for the rest of the quilt.
Shameless display of those past quilting escapades:
Cotton Candy Sue