I use red wine vinegar. Any kind that you like would work, I suppose. Just avoid balsamic, which has sugar in it.

I dump all the fixing into an old empty spice bottle and take it with a salad to work every day to lunch. I don't bother to mix it, just shake when I am ready to use it. I load up my salads with goodies like feta cheese, olives, avocado and/or smoked salmon, etc. Lunch has become about my favorite meal of the day!