What does falling off the wagon into a vat of alcoholism mean?

Answer 1

A double metaphor. To be on the wagon means that you have given up drinking alcohol. A vat of Alcoholism suggests a lot of alcoholism (a vat is large) and at the same time the word vat is usually applied to large containers of alcoholic drinks. To sum up it means going from one extreme of not drinking at all to the other extreme of heavy drinking.