Right. LV is saying that camera footage shows that both items were placed in one box at their distribution center.
A couple of thoughts;
So, happy it's not LV staff, in which case LV need to open a case with whichever courier they used to send it.
You will need to contact the police - advising LV you are doing this. I would guess the police would want to see the security footage showing both items placed in the box you received. Of course, we need to hope that the film also captures a continuous and uninterrupted feed which also shows the box then being picked up by the courier - with no break in which someone could open and rebox the package.
Do you still have the original packaging? If you do it would be very helpful as wouldn't it need to show the weight of the package?
Good luck with this one.