[SP] Retrospective
OK, this is much simpler than everyone thinks. A .vpk file is only a compressed format that the game uses, but if you open it up with GCFScape you'll see the folders inside (maps, materials, models, scripts). Just extract this content to whatever location in your pc; then drop them into portal2/portal2 folder and every file will be placed in its right location (say yes when it asks you to combine folders or overwrite files). Now you only have to run the game, open the console and type "map sp_retrospective" and it should play fine! 
(apart from the .vpk into this file "retrodist.rar" there are 2 folders ("media" and "resource") that should also be dropped into portal2/portal2 in order to run this map correctly...)
useful tools and stuff here on TWP 
[spoiler]
[/spoiler]
This is one of the best puzzles ever made. Nice! I had some problems starting it, but when I downloaded GCFScape, I easily extracted the .vpk folders to the portal 2 directory. The ending was epic! I liked the edgeless companion sphere and the green fizzlers very much. Did you make the 'the lemon is a lie' and 'know your lies' poster? Good one!