If your video quality looks fine before encoding, then you don't need to do ANYTHING in the avisynth script.
Export from vegas with any lossless codec such as huffyuv, lagarith, or UTvideo. If you are unsure, you can't go wrong with UNCOMPRESSED.
You can keep the audio when you export, just make sure its uncompressed.
Load it into zarx264gui (make sure you have the latest one:
http://amvhell.com/stuff/zarx264gui/) and press the encode button.