Can anyone guide me, on what should I do in order to make the vault app like the calculator app vault. I already know the lock but the adding files like photos, video and docs in is the one that I didn’t know.
MAYBE you can use the AppDirectory I think to hide and lock them. But I didn’t test it.
You can make a Folder in it and then in that folder, add an empty .nomedia file.
How to do this I’m noob I try other way to save file but its online storage and it work, but I want offline only and its hard for me to find tutorial of vault app. I need to make this to pass on deadline this coming 15 
Seems like a homework exercise. You should do this yourself instead of getting other people to help.
For our thesis proposal