Making vinaigrette is almost as easy as opening a bottle of salad dressing. It also keeps well in the refrigerator for three or four days, so there's really no reason not to make your own.
The traditional vinaigrette formula is 3 parts olive oil to 1 part wine vinegar. A little Dijon mustard helps the dressing stay mixed; salt and pepper are the only other required ingredients, though finely minced garlic, shallot or fresh herbs are classic additions.
You also can vary the oil and vinegar by using nut oils instead of olive and balsamic or sherry vinegars instead of wine vinegar.
The classic way to make vinaigrette is to whisk together vinegar, mustard and seasonings, then gradually whisk in oil until an emulsion is formed.
There's also a simpler method: In a jar, combine vinegar, a spoonful of Dijon mustard and seasonings. Screw lid on tightly and shake vigorously until mustard is dissolved. Look at the side of the jar to check proportions, add oil and shake again.