Vinegar pie and buttermilk pie can both be categorized under the broad term "chess pie." Any sort of acid added to the eggs - whether citrus juice in this recipe, or buttermilk or vinegar in others - will help lower the temperature at which the egg will achieve a custardy consistency. So, in short, vinegar pie and buttermilk pie are types of chess pie. Vinegar pie gets its twang from vinegar instead of lemon juice.  Lastly, buttermilk pie usually contains evaporated milk whereas chess pie typically doesn't contain dairy.