Hey, I was playing Vice City today and I was hanging around Print Works. Many Vercetti gangsters hang around there, so it is easy to hear dialogues. One of them has literally creeped me out, as a gangster said "I think you should tell him, Mario" or "Why didn't you tell him, Mario?" (I know that it ended "tell him, Mario", but I can't remember the start). As it is assumed that Mario actually knows about the whereabouts of Ricardo Diaz's Grave, I think that the gangster refered to it. This "him" is most likely to be Tommy. I don't know what to say about this.
Mario is a name for multiple gangsters who look the same basically, so he may be talking to himself, by the looks of the dialogue he seems to regret not telling something, interesting. You should also check out the sound files, to be sure you didn't hear it wrong.
And this is what happens when you don't play Vice City enough. There is nothing strange about this dialogue. The "Don't push the Mario man!" Means the guy means business. The "You better tell him Mario!" can mean anything. Maybe Mario forgot to pay taxes for the Estate? Nothing strange about it whatsoever. It's just random dialogue I have heard infinite times and it means nothing.