VPK (Valve Pak) files are uncompressed archives used to package game content. zip first and then open it with the decompression utility. Since VPK files are compressed using Zip-compression, you can extract the contents of the files using a Zip decompression utility, such as WinZip or 7-Zip.
Under the tf folder (or hl2 for Half-Life 2/Portal), find the file ending in _dir.