When I started this project I did not have much experience and understanding of how these games work. So my story isn't based off these "Porn Logic" and when it gets included can seem a bit off at first... I also am aware I don't explain things the best as I won't spoil the story and can only hint so much.
The Vault is extra scenes and images earned. Also contains replays of sex scenes. This is accessed from the Main Menu.
Just like your foot note says. Most people see NTR as really cheating... I do not have this tag for a reason as I am not a fan of this. Now according to some NTR in regards to sharing them with others is fine with me and will be included as normal. I have plenty of relationships that can lead to this happening or not. The one thing that does spoil the story, if you commit, they wont cheat on you. To conflict this, you shouldn't cheat either... if you do well, won't be long till you see "Single" again for your status
There are 4 paths in the game...
Love (Romancing any girl solo or throuple)
Lust (Open to others joining, also limited to whom)
Corruption (The dark path)
Corruption with NTR (The darkest path and no choices)
I hope that helps answer some queries.