wouldn't it depend on the fabric?
I can't imagine vinegar doing much for nylon, but maybe wool, as the acid would tighten up the fibers and 'crisp' it back up a bit? I'm guessing here, but using chemistry you should be able to find the right compound to soak a fabric in that will clean, tighten, relax or whatever, the fibers used in that fabric. What's the stuff you're trying to freshen made of?
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