This is a slice of vinegar pie prepared by food blogger Southern Bite. It's a type of pie sometimes called "desperation pies" associated with by not necessarily originating in the Great Depression, when hard economic times required creative substitutions in popular foods. In this case, when a pie requires a certain tartness but citrus juice isn't available, vinegar will do.
This pie is one of 18 old fashioned and largely forgotten pie flavors rounded up by The Takeout. Others include water pie (another desperation pie), mock apple pie, pineapple pie, and grasshopper pie, the last of which is named for the cocktail, not the insect.