The meaning of the term ἐπιστάτης, master, is seen in the LXX where is used 8 times:
And he set over them task-masters, who should afflict them in their works; and they built strong cities for Pharao, both Pitho, and Ramesses, and On, which is Heliopolis. (LXX-Exodus 1:11)
And the accountants of the race of the children of Israel, who were set over them by the masters of Pharao, were scourged, saying, Why have ye not fulfilled your rates of brick-work as yesterday and the third day, to-day also? (LXX-Exodus 5:14)
besides the rulers that were appointed over the works of Solomon, three thousand six hundred masters who wrought in the works. (LXX-1 Kings 5:16)
And they took out of the city one eunuch who was commander of the men of war, and five men that saw the face of the king, that were found in the city, and the secretary of the commander-in-chief, who took account of the people of the land, and sixty men of the people of the land that were found in the city. (LXX-2 Kings 25:19)
And Solomon gathered seventy thousand men that bore burdens, and eighty thousand hewers of stone in the mountain, and three thousand six hundred superintendents over them. (LXX-2 Chronicles 2:2)
and they brought thither the first-fruits and the tithes faithfully: and Chonenias the Levite was superintendent over them, and Semei his brother was next. (2 Chronicles 31:12)
The Lord has made thee priest in the place of Jodae the priest, to be ruler in the house of the Lord over every prophet, and to every madman, and thou shalt put them in prison, and into the dungeon. (LXX-Jeremiah 29:26)
and one eunuch, who was over the men of war, and seven men of renown, who were in the king's presence that were found in the city; and the scribe of the forces, who did the part of a scribe to the people of the land; and sixty men of the people of the land, who were found in the midst of the city. (LXX-Jeremiah 52:25)
None of the uses are vocative; however, the meaning is straightforward. It describes someone with secondary authority. For example, not Pharaoh, but his task-masters; not the king, Solomon, but one he appointed. What is described is someone with lesser authority.
Therefore, "master" as an vocative translation should be understood as a form of address to someone who is superior but whose authority is limited especially relative to "lord," a title given to the one with the highest authority.