-SOLVED- extract content of pak_01dir.vpk with GCFscape
I have created a backup of pak_01dir.vpk and opened it with GCFscape.
OK, I have solved that problem. Now I have the vmt and the vtf datas. But when I try to open them, Windows says, "side-by-side configuration is invalid".
A map pack coming soon. - click for more information
If you want I can sent you a zip with these files?
Since I have no idea what causes this problem (probably verifying game cache wouldn't hurt 
)
Forget it already. I solved the old problem and a new one occured. (Look at first post)
A map pack coming soon. - click for more information
The side-by-side thing has something to do with Microsoft's .net framework, which is a programming component you need for both GCFscape and VTFedit. It usually means that something is not installed correctly.
Try reinstalling (and, for good measure, re-downloading) the .net framework and see if that fixes it.
(Use this link if you have 64-bit Windows.)
Other Portal 2 maps: Medusa Glare
Portal 1 maps: Try Anything Twice | Manic Mechanic
I already installed .NET framework 4.0
EDIT: Problem solved. VTFedit was wrong installed. I installed it correctly afterwards. Windows though, linked all .vmt datas with the old not-working VTFedit.
Thank you for your help. 
A map pack coming soon. - click for more information
That should also work; it's the same one I have installed.
Are you able to open other textures with VTFedit? If not, then try re-installing 4.0.
edit: There's actually some talk about this error on the download page of VTFedit. (Click the comment links.)
Other Portal 2 maps: Medusa Glare
Portal 1 maps: Try Anything Twice | Manic Mechanic