Formerly, a party in which everyone brings their own alcohol, which upon entry is taken and poured into a large vat with the rest of the booze. This concoction is served in cups from a tap at the bottom of the vat, and has become less popular over the years.
More recently, a vat party is when several attendees bring a supply of recreational pills to a party, preferably of several kinds or types, and the pills are all mixed into a bowl where adventurous attendees can investigate potential combinations for a particular effect, or discover their own. Many people die at such parties, especially when mixing the pills with alcohol.
Five people in my family had surgery this past year and I'm gonna host a hell of a vat party with their leftover scripts!