Now, many people wondered can downgrade v7.5 vtf files to v7.2 vtf files. Well, its possible. I'm going to teach you how to downgrade vtfs. I uploaded an addon called portal 2 portal gun skin for portal. I used the same way downgrade 'it' to be compatable to Portal.
VTFs from Portal 2
IQ above 5
Step 1: Open up VTFEdit and load file.
Step 2: Click 'Tools' on the top then click 'Convert Folder' and wait for 'Batch convert' window to pop up.
Step 3: Put the VTFs to a new folder.
Step 4: Click the '...' button next to the input folder box then select the folder with the VTFs.
Step 5: Repeat step 4 but on output folder.
Step 6: Check 'To' then select 'tga' and make sure the text box's content is *.vtf then uncheck 'Create VMT files'
Step 7: Click 'Convert'
Step 8: Check if TGAs are in the dir of the output folder.
Step 9: Go to VTFEdit.
Step 10: Repeat step 2
Step 11: Type in which folder the TGAs are in the 'input folder'.
Step 12: Type in which folder the VTFs are in the 'output folder'.
Step 13: Make sure 'Create VMT files' is unchecked
Step 14: Click 'Convert'.
Step 15: Done