Heu, omnes! I've been kind of embarrassed about this for a while, but I don't feel totally confident that I understand the vocative forms.
Naturally, any second declension masculine noun goes from -us to -e.
Also, nomina ending in -ius, and filius, become simply -i (as in, Quid petis, o fili?, or, Multas gratias, Tuli!)
And as for nouns that end in -eus, they also become -i, correct? So what otherwise might be mee amice ...