When I make salad dressing, I usually don't expect my vinaigrette to emulsify particularly well. I don't do any of the steps described in this question about vinaigrette emulsification, such as drizzling the oil into the acid slowly with much stirring.
However, last week I decided to try to make a vinaigrette using some aged balsamic vinegar that I just bought. I've used aged balsamic in the past and had the same experience that I normally have with the vinaigrette -- that the mixture doesn't stay together -- I need to stir just before dressing the salad.
I poured olive oil into my bowl, added my new vinegar, and some salt, pepper, and herbs -- and almost immediately, the mixture turned into the texture of thin mayonnaise. No matter how much oil I added later to thin it down, it didn't break the emulsion. (I ended up saving it as a sauce for chicken, since I don't like thick salad dressing.)
Why did this emulsify so well? Could there have been something about that vinegar? (It's aged and thicker than most, but not so much that it compares unfavorably with other aged balsamic I've bought. It's just vinegar -- no added ingredients.)