Set up your pictures how you want it. Meaning, start with a plan. Use Photoshop or something similar to where you can take one picture and make it smaller and paste it on another picture and so on and so forth. Once you get all the pictures you want pasted on the other pictures, save them and import them into Vegas.
Now, set up a 3D space and hang your pictures appropriately and tweak as necessary to get the fade or "pull through" that you want.
you can di it all in vegas by using the pics, motion and corss dissolve transitions, but it is harder than just using After Effects. I did this sort of thing just a few weks ago and tried it in Vegas, but chose to do it all in AE because of the AE camera track for easier keyframing of motion and it's 3d.
I could be done in Vegas but the amount of time and effort required on these types of projects can be HUGE. I've done work like this and take it from me this is not what Vegas is made for, though it's able to do it, you will get very frustrated doing it. I prefer Combustion to AE (can't stand the AE interface) but that's just me, I was able to just open Combustion and start figuring it out, AE was a maze to me (I don't think like adobe does).
3D placement in Vegas is very time consuming because there is no good way to get one overall view that you can move things within, you have to view it one place and individually move them from another window that does not give you any info as to where the other pics are within the composite.
Can be done, but get a compositing software, that's my suggestion.