Place the vinegar and the mustard in a medium-sized bowl and whisk together. Whisk in the salt, then slowly whisk in the oils. Your vinaigrette is likely to be quite thick; if it is too thick for your liking, whisk in some warm water. Taste for seasoning, and adjust accordingly. Transfer the vinaigrette base to a bottle or container of your choice, and reserve for those salad moments. The vinaigrette doesn't need to be refrigerated, unless you don't plan to use it.