For this dressing, you want to use a good quality red wine vinegar. There are a lot of cheap ones out there, and the taste of most of them is sour, rather than tart, and metallic, aggressive, even bitter. Shun them, as all they will do is ruin your food. Since this dressing is so simple, there is nowhere for an inferior vinegar to hide. While you want to use extra virgin olive oil here, save the heavier Tuscan or Olio Nuovos for other dishes. Something lighter is called for here so the flavor of the beans will shine through. Reducing the vinegar concentrates the flavor and gives it a bit more body, like a balsamic vinegar. I give two measurements for the oil here. The first is for what is referred to as a “slack” vinaigrette-meaning loose, as it will not stay well emulsified due to the ratio of oil to vinegar. This is what you use for salads using vegetables such as beans or carrot, where you dress blanched vegetables while still warm, and is more tart than most salad dressings. It is good on cabbage salads, too. The second is for leafy salads or things that are dressed just before serving, where you want the dressing to cling. It is milder and will stay emulsified better.