You shouldn't pour vinegar into someone's drink.
Nick's friend Matt ordered a drink and then Matt went to the bathroom while they are at a restaurant together.
Nick wants to play a prank on Matt.
Nick takes Matt's drink and hides it.
Nick pours vinegar that is on the table into Matt's drink.
Matt will be able to get his drink back and not have to order another one.
Matt will have to order a new drink and spend more money.