I soaked mine (in just plain hot water, didn't know about the vinegar idea) a couple of days ago because both my mats were yucky. My mats are too big to lie flat in the tub, so I just let them curve, and swooshed them around every now and then, and scrubbed them with an old toothbrush (but I think the vegetable brush idea is better). I was trying to get upholstery fabric threads out of one mat, and school-glue-plus-fuzzies off the other. Both mats feel much better, though I couldn't get all of the upholstery fabric fibres out of the one. They did not stay curved after I removed them from the water, but they were still warm as I dried them with a towel and so at their maximum flexibility...
I use the markings on my mats to measure most of my cuts, so some of the grooves in them are never going to come out, but I did find that soaking them improved their texture markedly.
Alison