Please keep in mind that the camera roll is system integrated and the vault is not.
So you can not encrypt your photos directly in a vault.
Photos are always stored in the camera roll first.
For Android you can automatically add new photos from your camera roll to a vault, in iOS this feature is not jet implemented.
No, the mobile apps do not work with mega (see here).
But as far as I can read in this board, backblaze should work, at least for android (see here)
Any provider that offers a WebDAV interface, or one of the native supported ones.