Grandpa Solberg grew up in Norway and emigrated to western North Dakota in the early 1900s. Gustav's childhood diet, like that of all coastal Norwegians, included fish aplenty. And, like all immigrants, he brought his dietary tastes with him (unfortunately, he didn't find a ready source of seafood on the Dakota grasslands).
But when Gus could get fresh fish, one of his favorite meals was fish soup ("fisk suppe") made the way it was done in the old country. I believe this closely replicates the original, but recipes from his era often lack much in the way of detail ("a little" flour, "some" fish, etc.).
This is health food; Grandpa Solberg lived in fine fettle to just a few months short of age 100!
Fish. Use firm white fish such as cod, haddock, hake, or pollack. Salmon is really good too. If frozen, remove the fish from the package and thaw in a bowl of cold water. Remove any bones and skin.
Vinegar? Use either plain vinegar (or white wine vinegar) or buttermilk.