How to get mustard stains out?

I am working with what I have, I rinsed with cold water, made a baking soda paste to scrub it with. And a second paste to let it soaked in for like 30 min. I've heard that you can use vinegar in combo with the baking soda. I only have apple vinegar, will it work the same? I am very upset that some idiot put a pate of hot dogs with ketchup and mustard in my chair a few seconds before I sat down. I always look before I sit to avoid things like this! My shorts are made out of a spandex/nylon type material, the ketchup came right out after the scrub. But the mustard lingered. So after I am finished letting the paste soak in should I rinse with the apple vinegar and follow with cold water?