the vocative: ''denoting a case of nouns, in some inflected languages [notably Latin], used when the referent of the noun is being addressed''

eg. 'quo vadis, domine?' and not 'quo vadis, dominus?'


It doesn't generally exist in modern Russian except in relic form, usually connected with worship: 'боже', 'господи' etc.

And, since 'the referent of the noun' in this case just ain't there, why not focus your efforts on learning something more useful? Far-eastern fiscal vernacular, perhaps...