When exporting a texture with WW Pro, you have many options that can affect the quality and filesize of your VTF files. Here are a few quick tips:
DXT1 -- Use this on your textures that do not need to use the alpha channel (keeps file size small)
DXT3 -- Use this for textures that need to use the alpha but don't need fine alpha transitions (things like fences, grates, etc). Good candidate when using Alphatest.
DXT5 -- Use this when the alpha must transition smoothly (large file size).