Can anyone pls tell me how to finish his unmarked quest? I wanna see his full shop, so dont want to turn him in, if i make a deal with Keller, then return to Contreras he just says sthing like "Kay, whatever" and the quest is not finished, Keller doesnt say a thing either anymore. If i kill Keller NCR gets angry at me (cant even get back into Mccarran, i get shot at sight, despite having a previous rep of idiolized), which i really dont want. So how could i finish this quest?

Speak to him and then shoot him and your rep shouldn't change with the NCR. I experienced a bug after this where I would go back to Contreras and I could only tell him that I made a deal with Keller, even though I didnt.
You could be experiencing one of this game's many many many bugs