The vocative case which I and other Russians I've witnessed use is prepositional vocative эй.
эй which is based on "hey" and can be used in the same manner.
эй Стас, Диана любит тебя "hey Stas, Diana loves thee".
It is somewhat colloquial but is also effective;
people will understand what you mean.
It is also more useful than the declensional vocative(s), since it can be used with any noun.