Pour the vinegar into a small glass jar.
Using a small sharp knife cut the vanilla bean and then split the halves lengthwise. Open each half and scrape the seeds from top to bottom into the vinegar, as thoroughly as you can. Add the vanilla pieces to the vinegar as well.
Put the lid on the bottle and close tight. Shake it for a good 30 seconds. Leave it at room temperature. For the first couple days, shake it a couple times a day. Let it sit for at least a week before using.