[vimeo.com]* is block by GWF
but [pdl.vimeocdn.com] is not
"http://pdl.vimeocdn.com/83856/004/181789841.mp4?aktimeoffset=0&aksessionid=7d9842dfdc10560dca5201dcb19d187e&token=1376301058_099a7228cc11d30dde9e9ffed42d9bd6"

so I want to use proxy_A to connect to [vimeo.com]* parse video information or video url
and down the video through proxy_B for [pdl.vimeocdn.com] or just download directly.

This seems like a corner case. Most servers require all requests from the same IP address.

can't you just support proxy.pac

[en.wikipedia.org]

Seems interesting, maybe it can be supported in a future version.