vocative?

[Yoshimasa Tsuji]

Dear experts,
I wonder if accusative case form is used for vocative in the Russian
language. I have long thought only the nominative case form is used
for that purpose.

Here's my recent experience: As I was sitting in a marshrutnyj taxi,
which is just a micro bus, yesterday, a chap suddenly found a friend
in the street and invited him to join him in the bus. On that occasion
he cried, "Va-le-ru!" a couple of times. I was puzzled by thinking
why he didn't say "Valera!"  Valera because he was wanted? Or is that
a Ukrainian vocative?

I have asked a couple of my Russian friends for explanation, and got
the same answer so far: "illiteracy".  As I don't think there exists
such a thing as illiteracy among language users, I have felt it
necessary  to put you a query.

Thank you.

Tsuji