Vinaigrettes and dressings are the best way to make salads less dull (and in some cases less frightfully healthy). Most vinaigrettes are naturally vegan, although in some less-than-gourmet French restaurants, there's a bad kitchen habit of thinning down some mayonnaise or milk thing and calling it a vinaigrette. It's therefore worth asking when ordering a salad what exactly goes into the vinaigrette. You can usually recognise these awful dressings when they arrive. They look kind of like the white goop here. I will make no other comparisons.
A rebellious word on emulsification
Unless you're planning to keep your vinaigrette for a long time, it doesn't really matter what order you combine the ingredients in. Don't waste time trickling in oil as slowly as possible, or whisking like a woman possessed. As long as you have fat, acid, and a binder like mustard, miso, tahini etc. you'll be fine. If it separates, it's not the end of the world - just shake it up a bit.
Easy, classic vinaigrette
One part red wine vinegar, two parts olive oil, one part dijon mustard, salt, pepper.
Shake everything together in a jam jar. Or in the mustard jar, as below, if you're nearly out. A neat way to use up the last bits left.
Play around with the proportions to find your perfect vinaigrette.
One part raspberry vinegar, two parts olive oil, one part whole-grain mustard (moutarde à l'ancienne), salt, pepper.
Do your jam jar thing again.
2 Tbsp peanut butter, 1 Tbsp rice wine vinegar, 0.5 tsp sesame oil, 1 tsp ginger paste, 1 Tbsp soya sauce, 2 Tbsp warm water, hot sauce to taste.
The shaky shaky method doesn't work so well here, due to the thick peanut butter. It's not much more effort though - stick everything in a mug and whisk with a fork until well combined. Add more water if you want something thinner, less if you want more of a dip.
Despite being (in my opinion) vastly over-used, balsamic vinegar does have its place in vinaigrettes when used with caution.
Make the "Easy, classic vinaigrette" above with balsamic vinegar rather than red wine vinegar and it goes beautifully in a lentil and carrot salad. A couple of drops of liquid smoke, strange as it sounds, works really well here too.
Or, if you're making an antipasti salad with already-oiled ingredients like sun-dried tomatoes or roasted peppers in oil, marinated mushrooms, olives, etc, then all you need is a small splash of balsamic vinegar, with no extra oil.
Look how sad these avocado halves are. I think they want walnut oil and red wine vinaigrette, with plenty of mustard to thicken...