Some remarks on the use of the vocative case in modern Greek.
Nouns and adjectives (from now on n & a) in -ος form their vocative in -ε. This is the general rule but in reality things are more complicated:
Feminine n & a in -ος
a. Most islands and some more place names have feminine names in -ος (Ρόδος, Μύκονος, Νάξος, Πάρος, Κύπρος..). Their vocative is always identical to nominative: Κύπρος! (never Κύπρε)
b. Probably because of the above some grammarians think the vocative of all feminine n & a in modern Greek should be formed in -ος always. Some words indicating professions and functions are included here, like υπουργός (minister), πρόεδρος (president), πρωθυπουργός (prime minister) etc. Some people will say thus "κυρία υπουργός", (Mrs. President), obeying to the rule above but I think this is a violent "modernization" of the language which contradicts what people say and are used to. I mean it sounds much more normal to a native to hear and say "κυρία υπουργέ", which is also the rule in ancient Greek, so I don't understand why we should be forced to change that to "κυρία υπουργός", which sounds bizarre to everybody. I would thus prefer "κυρία υπουργέ".
Human names in -ος
Here things are a bit complicated. I will try to give general hints:
a. Nicknames, common names and family names of 2 syllables, form the vocative in -o, just like the accusative:
Γιώργο! George
Μάνο! Manos (nickname of Emmanuel)
κύριε Τέγο! mister Tegos (a family name)
κύριε Σώρο! mister Soros (same)
b. Formal names, ancient names and names of 3 or more syllables, usually form their vocative in -ε:
Αλέξανδρε! Alexander
Πάτροκλε! Patroclus
Αθανάσιε! Athanasios
Γεώργιε! George (the full and formal form of the name)
κύριε Παπαδόπουλε! mister Papadopoulos
κύριε Χαράλαμπε! mister Charalambos
c. Family names of 3 syllables or more can form the vocative either way. Sometimes it's a matter of what sounds better. However I think that the vocative in -ε always sounds OK, while forming it in -ο can be a risk for you. Besides the vocative of these names in -o is just one more wrong perception of "modernization". I would advise you to use the vocative in -ε here:
κύριε Παπαδόπουλε!
κύριε Πάγκαλε!
κύριε Δαμιανάτο / Δαμιανάτε!
Calling someone loud in everyday speech
In everyday language and among people who know well each other, when we want to call someone who doesn't pay attention to us on that very moment, for example someone who walks in front of us on the road, someone who looks to another direction etc, we sometimes use the nominative:
σκοπός! guard
ταχυδρόμος! postman
Παπαδόπουλος! Papadopoulos (a family name)
However we don't do that when the person gives his attention to us. I.e. when the guard will finally pay attention to us and we will start a conversation with him, we will call him "σκοπέ"
Nouns denoting profession or function, ending in -της
In ancient Greek these nouns formed their vocative in -τα and in modern language people still use this ending, thanks to the influence of καθαρεύουσα. The ancient ending is used almost always by everyone, while the demotic type in -τη is not preferred. The commonest nouns in this category are:
καθηγητής - καθηγητά (professor)
πρεσβευτής - πρεσβευτά (ambassador)
διοικητής - διοικητά (commander)
διευθυντής - διευθυντά (director)
σκηνοθέτης - σκηνοθέτα (film director)
κυβερνήτης - κυβερνήτα (governor)
The archaic ending -τα is preferred by the people probably because it sounds more honorific, while the modern ending would sound disrespectful. At the university we always addressed our professors "κύριε καθηγητά" and at the army when talking to the commander and the sub-commander "κύριε διοικητά" and "κύριε υποδιοικητά"