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VIP Video Converter?
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2 reviews of this app found across the web:
Well. The video codec looks completely standard, and the "YUV 4:2:0 P" is a reference to the colorspace in the video, which I assume Vegas 12 also supports.
However, DTS audio has never been supported in Sony Vegas Pro: http://www.vip-video-converter.com/articles/sony-vegas-pro-14-supported-formats-convert-video-audio/ (Couldn't find an official source with this documentation)
The process you're doing with xvid4psp is commonly known as remuxing. Try to do it again, but only remux (aka. stream copy aka. passthrough) the video codec. For the audio, try to convert it to a supported audio codec like FLAC or AC3.
Handbrake isn't a good option, because it doesn't support video passthrough, and it would be a shame to re-encode the video if it's not necessary. But if the above suggestion doesn't solve anything, you
A quick google search shows that you could convert your swf files to mp4 or other format.
http://www.vip-video-converter.com/articles/how-can-i-convert-swf-to-mp4-using-vlc/
There are other methods available, I have not tried any of them. Make a copy of one and give it a go.