Mason Dixon KAL

I remember reading that the colors of PnC/SnC cotton could be made somewhat more colorfast by washing in vinegar before the first use. Does anyone have specific instructions for how to do this? Soak in pure vinegar then rinse in water? Detergent or no? Or is there a water/vinegar ratio to use when washing in detergent? Or something else? I love the bright colors of my PnC/SnC warshrags, bibs, baby kimonos, etc and would love to keep them looking like that as long as possible.