It doesn't use the vocative case, but rather the nominative. Thomas didn't address Jesus, hence there is no "You are..." before "my Lord and my God" and why no one translates it in such a way. John didn't write John 20:28 even remotely thinking that Thomas was addressing Jesus, but rather remembered it as Thomas making a declaration. In other words, Thomas thinking Jesus was his God was the furthest thing from John's mind. There is a lot of evidence for John not believing Jesus is God around the New Testament, on that note.
Are you still using that debunked claim. I told you previously the vocative case was replaced by the nominative case with an article
THE VOCATIVE CASE
There is a fifth case that really has no sentence slot to live in. Maybe it isn’t a true case. But it does seem to have a discrete ending sometimes. It did not show up on the article chart because this case has no article.
It is the way you spell someone (or something) when you are talking directly to him (it):
πάτερ, ἄφες αὐτοῖς (Father, forgive them.)
Here, “Father” is written in the Vocative case.
New Testament Vocatives are rare and those with discrete endings differing from the Nominative endings are extremely rare.
Edward W. Goodrick, Do It Yourself Hebrew and Greek: A Guide to Biblical Language Tools (Grand Rapids, MI: Zondervan, 1980), x.
also
Direct address
The vocative case in Greek is used for direct address123.
It is used when addressing someone directly, calling out to an individual by name, or welcoming or referring to them by name3.
The vocative case is usually identical to the nominative case in form3.
It is sometimes accompanied by the particle "o" to add emphasis or emotion3.
Each declension has its own vocative form4.
My Lord and my God (ὁ κυριος μου και ὁ θεος μου [Ho kurios mou kai ho theos mou]). Not exclamation, but address,
the vocative case though the form of the nominative, a very common thing in the Koiné. Thomas was wholly convinced and did not hesitate to address the Risen Christ as Lord and God. And Jesus accepts the words and praises Thomas for so doing.
A.T. Robertson, Word Pictures in the New Testament (Nashville, TN: Broadman Press, 1933), Jn 20:28.
28.] The Socinian view, that these words, ὁ κύρ. μου κ. ὁ θεός μου, are merely an exclamation, is refuted—(1) By the fact that no such exclamations were in use among the Jews. (2) By the εἶπεν αὐτῷ. (3) By the impossibility of referring ὁ κύριός μου to another than Jesus: see ver. 13. (4)
By the N.T. usage of expressing the vocative by the nom. with an article. (5) By the utter psychological absurdity of such a supposition: that one just convinced of the presence of Him whom he deeply loved, should, instead of addressing Him, break out into an irrelevant cry. (6) By the further absurdity of supposing that if such were the case, the Apostle John, who of all the sacred writers most constantly keeps in mind the object for which he is writing, should have recorded any thing so beside that object. (7) By the intimate conjunction of πεπίστευκας—see below. Dismissing it therefore, we observe that this is the highest confession of faith which has yet been made;—and that it shews that (though not yet fully) the meaning of the previous confessions of His being ‘the Son of God’ was understood. Thus John, in the very close of his Gospel (see on vv. 30, 31) iterates the testimony with which he began it—to the Godhead of the Word who became flesh: and by this closing confession, shews how the testimony of Jesus to Himself had gradually deepened and exalted the Apostles’ conviction, from the time when they knew Him only as ὁ υἱὸς τοῦ Ἰωσήφ (ch. 1:46), till now when He is acknowledged as their LORD and their GOD.
Henry Alford, Alford’s Greek Testament: An Exegetical and Critical Commentary (vol. 1; Grand Rapids, MI: Guardian Press, 1976), 912.
maybe you should avoid making arguments about things of which you have no knowledge
