Haha I decline your proposal not to get into that debate: the vocative is a grammatical case. Maybe not every noun can be put in the vocative, but it’s definitely one of the cases. Even the locative is a case, even though only a couple of nouns use it.
The Ohio State University seems to think these are both cases: https://classics.osu.edu/Undergraduate-Studies/Latin-Program/Grammar/Cases/latin-case .