D.o.C. info plz

I beat ff7, got all the materia, lvl 80 sumthin, ultima weap, beat weaps except emerald, got all that fun stuff, and i loved the game. I have still yet to watch A.C. (kinda sad cuz i'm living in Taiwan), and haven't really researched DoC yet. Could sum1 tell me the secret ending that every1's talkin about, and what we know? I know vincent's the main character now... plus, how can this be a shooter unless u only play as vincent? that' d suck if u can only play as 1 character, this defies FF tradition