One sees a lot of discussion about a "new vocative case" in modern Russian; however, I am not asking about that.
My question is this -- in the instances where only one of the six standard cases is used, which of the six is used to express the vocative?
My first guess would have been nominative, because they are similar. However, in German, for many expressions of a seemingly vocative nature, accusative is used instead: "guten Morgen!", for example. And actually the same seems to be true in English (although there one would describe it as the objective case) -- "Lucky me!".
So, when an entirely different construction isn't used, which of the six standard cases are used to replace the vocative in modern Russian?