What rendering format offers the best video quality on sony vegas pro 9?

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avi is the best choice. but youtube has better support for FLV video file , if you cant upload your video to youtube ,you need to do converting your file into FLV with some media converter.

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In my view, mp4(avc video codec, aac audio codec) video format is the best choice for Sony vegas pro 9, and for youtube, 1280×720 16:9 MP4(h.264 video codec, aac audio codec), or 640*480 4:3 MP4(h.264 video codec, aac audio codec), or normal FLV videos, all of them are the best video formats for Youtube. So, I think, you just need a right converter to convert. I am using RZ Video Converter which can convert any videos or audios between different formats with good quality, such as convert any videos to sony vegas mp4 videos, or convert any videos to the best youtube format videos, etc. Try it by yourself.Yahoo or google search and download rz video converter. Hope it can help you.

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AVI would offer the best video quality.But, for youtube, you have better to offer the flv video.

You can use iSkysoft Video Converter. It can convert your video to FLV/WMV/AVI/ASF and other popular formats such as: H.264/MPEG-4 AVC, MPEG-1, MPEG-2, 3GP, 3GPP, DivX, Mov, M4V, DivX, XviD, ASF, SWF, VOB, DV, Super VCD(NTSC, PAL, SECAM), VCD(NTSC, PAL, SECAM) , DVD(NTSC, PAL, SECAM) and HD video.
It is also multifunctional such as video trim, effect, crop and watermark.

For more information, here:
Hope it helped.