Well:
Slide
The video of Madonna is untouched (exc some effects) and positioned as the main track (in Vegas that's the video which is positioned lowest).
From Timbaland and Justin's video I marked the part I needed, inserted 2 new video tracks and put that same part in both tracks (they have to start simultaneously). Than, I've masked (in Pan and Crop) one half of first one of them and second half of second one (just like You draw a line in the middle). After that You go to the Position track in "Pan and Crop" and adjust both "squares" to move in time (left to the left side and right to the right). If You've never done that before You'll find this tutorial a bit confusing (i should make a vid tutorial but I don't have time) but I'm sure if You play with this a bit You'll get it!

As for Justin in boxes... I took the part I needed of Justin's video and put them in 3 new video tracks (and did that every time Justin appears exc when there is only one pic). I've masked all 3 videos into a box of a size of a box in Madonna Video and put (with position in Pan and crop) every part in its own box.