In post production you may be messing the footage up. Not knowing how to render footage can really compress it and make it look really bad.

There are tutorials on YouTube on how to render on Sony Vegas. I use SV Pro 8, and it works great for me. I'd say for the most part when you go and render (in say MPEG2 or AVI) click the CUSTOM button. Usually the image quality is preset to like 50%. Set it to 100% and see if that helps.

Also, if you shoot in 16:9, the default for MPEG is to letterbox it. Again, click custom and set that to 16:9.

If you are saving for YouTube, I found that WMV gets better results. Click custom and set it to:
Mode Quality VBR, Animation 320X240 and quality to 100%.