Vinaigrette is a simple salad dressing made from an emulsification of oil and an acidic ingredient, often with other flavourings added.
The traditional proportions of vinaigrette are three or four parts oil to one part acid (usually vinegar or lemon juice), plus salt and pepper and maybe mustard and/or garlic. But in this crazy modern world, where cats lie down with dogs and up is down, any old proportions for this concoction that suit your taste will do.
Although olive oil is a classic component of vinaigrette, you can also try expensive but tasty walnut oil, grapeseed oil, or sesame oil (which, because of its strong flavour, is best mixed with another oil), or something much blander, like canola or safflower oil.
For the acidic ingredient, try a lovely red or white wine vinegar, or mild balsamic vinegar, or, if using sesame oil, follow the Asian theme and use rice vinegar. Lime juice is another good one to try.
Flavourings? Dijon mustard, fresh herbs (tarragon, basil, or parsley are classic), mayonnaise, yogurt, buttermilk, olive paste, pesto, tahini...the possibilities are endless. And if your acid has a strong flavour, be sure to add a little honey or apricot jam to the mix to balance the sourness.
First, mix together the acid ingredient and flavourings. Then add the oil slowly, whisking (if in a bowl) or shaking (if in a closed container like a jar) after each addition. Or mix them in a blender and add the oil in a slow steady stream with the motor running. Adding the oil slowly and mixing it in well will help keep the dressing emulsified until you can use it for dressing your greens.
A vinaigrette will keep for a week or two in the fridge. Whisk the dressing briskly before adding to a salad, and don't use more than is necessary to lightly coat the salad.