The absolutely basic formula for a vinaigrette dressing (or French dressing, if you prefer,) I am sure you will agree, is three parts oil to one part of vinegar. So simple, so ubiquitous, it seems like it must have been around forever, yes? And just as obviously, it must have originated in France, yes?
The word vinaigrette comes from the French vinaigre, for vinegar, so presumably this was the key ingredient. The first entry in the Oxford English Dictionary suggests that this was the case in England too, as oil is not specifically mentioned:
1a. “A condiment prepared with vinegar.”
First use in English cited as being in 1699, in John Evelyn’s famous work, Acetaria – “Cucumber, ... tho' very cold and moist, the most approved Sallet alone, or in Composition, of all the Vinaigrets, to sharpen the Appetite.
1b. “Now spec. a dressing of oil and (wine) vinegar, sometimes with herbs (esp. tarragon), used esp. with salads and cold vegetables. In full vinaigrette sauce (also French sauce vinaigrette). Also vinaigrette dressing.
First use in English in this context is given as appearing in 1877, in Cassell’s Dictionary of Cookery:
“Vinaigrette, Sauce à la. This is a sauce much used in Paris for cold viands.”
Food was in fact dressed with oil and vinegar (by the wealthy, that is,) throughout Britain and Europe long before the relative amounts were quantified and it gained independent identity as ‘a sauce.’ No doubt one of the benefits, and perhaps the main impetus for the practice, was the preserving power of vinegar in the days long, long before refrigeration.
Here is a recipe for a ‘vinaigrette” from an early fifteenth century French manuscript referred to as Du fait de cuisine (with thanks to the excellent site maintained by David Friedman.)
Again, a vinaigrette: and to give understanding to him who will make the vinaigrette let him take pork livers and wash them and then put them on the grill over fair coals until they are cooked enough; and when they are cooked let him put them on fair boards and then slice them into little dice; and then let him take a great deal of onions and peel them and wash them and slice them very small and sauté all of this together in good and fair lard. And for the potage of the said vinaigrette let him take very good claret wine of the best which he can get according to the quantity of the said potage and put in what is needed of beef or mutton broth; and then let him take fair white bread and slice it into fair slices and put it to roast on the grill until it is well browned, and then put it to soak in the said wine and broth; and when it is soaked take spices: white ginger, grains of paradise, pepper – and not too much, a great deal of cinnamon as is necessary, and also salt, then pass and strain all of this through a strainer cleanly and properly, and then put it to boil; and, being boiled, throw in the said sautéed meat. And then serve it when it should be served.
In nineteenth century Britain, the oil and vinegar combination became known as French dressing or sauce. Here is the recipe quoted in the OED as the first known use in English (I suspect there are earlier uses, but that will take some searching.)
Vinaigrette, Sauce à la. This is a sauce much used in Paris for cold viands. Sauce à la vinaigrette is composed of salad oil, vinegar, finely-chopped parsley, and shallots, onions, or chives, with pepper and salt to taste. For those who have no objection to oil this sauce is infinitely superior to mere vinegar, pepper, and salt. It is suitable for every kind of cold meat, and especially for cold calf's head, and is admirable with cold salmon, turbot, or indeed any sort of cold fish. Hard-boiled eggs also eat extremely well with sauce à la vinaigrette; so do many kinds of cold vegetables, and especially asparagus; in fact, this is quite as often eaten cold as hot in Paris, and always à la vinaigrette. Cold artichokes are also very largely consumed with this sauce. When used with cold meat, and particularly with calf's head, the addition of a few capers to the sauce is a great improvement; and with cold roast meat a gherkin cut up fine is excellent. As this is a sauce produced almost entirely out of the cruet-stand, its suits well with our English habits. You rub up the salt and pepper with a little vinegar, then add as much oil as you please, with chopped parsley, shallot, gherkins, or capers, according to convenience or taste. Food Journal.
Cassell’s Dictionary of Cookery (1877)