According to Harold McGee, a vinaigrette is a water-in-oil emulsion, which means that tiny vinegar droplets are dispersed within the oil, while the oil is the continuous phase, meaning it coats each droplet and fills the empty space between the vinegar droplets. Compare this to, for example, mayonnaise, which is an oil-in-water emulsion with oil droplets suspended in water.
A vinaigrette is usually made by first adding the salt and vinegar to a bowl and allowing the salt to dissolve. Next, the oil is added. According to Michael Ruhlman in the book Ratio, the manner in which the oil is added determines the texture and longevity of the emulsion. If oil is added all at once and briefly whisked with a fork, the vinegar droplets in the oil remain relatively large and only temporarily combine. Adding the oil in a thin stream while the vinegar is being blended continuously with an immersion blender results in droplets of vinegar that may be as small as 3 thousandth of a millimeter across (Harold McGee) forming a thick and stable emulsion with a consistency similar to mayonnaise (depending on the water content).
The ideal ratio for a vinaigrette is 3 parts oil to 1 part vinegar or other acid such as citrus (Michael Ruhlman). 3:1 is roughly the ratio I use in this lemon vinaigrette, which also has the lemon peel added for additional flavor. I use a fork to briefly blend the lemon juice and olive oil, creating only a temporary emulsion. Alternatively, an immersion blender could be used to whisk the lemon juice while slowly incorporating the oil. When I make this vinaigrette with an immersion blender, I often add the herbs to the vinegar and salt mixture, creating a lovely pale green (and delicious) dressing.