Technically mustard is mustard and water. But vinegar and salt provides a shelf stable and safe product. Mustard by itself is antimicrobial and antiviral. The combination means you easily get 6-12 months in the fridge. To be clear, I did not mean to imply don't use vinegar. That's why I said, "Now continue with your favorite mustard recipe." I personally prefer apple cider vinegar with mother and let the mustard sit for at least 8-12 hours to ferment. Others prefer 2-5 days. For chemistry reasons I don't understand, mixing with cold water activates the spiciness of the mustard seed (especially the black; yellow mild and black is spicy). The longer you wait to mix with vinegar the spicier it becomes, up to around 15-20 minutes. The addition of vinegar halts the chemical reaction and locks in that spice level.