1) Vault infections are not permanent.
2) You must have missed a step in the process. Where did you get the vault mod from if I may ask?
3) Yes, however some service providers allow you to copy the video into your vault so that you can get the infections whenever you want.
4) No, it does not download any files on your PS3.
5) Yes, it only loads the videos from the server, it does not store them on your PS3's hard drive.
6) No, as the infections are not permanent.
7) It adds file size to the save data utility although I'm not too sure about that. Even if it does, it's an insignificant amount of space being taken up.

No, it is not dangerous to play with vault mods.
I hope this answered your questions.