Will this output video during post and in BIOs menu? Or only in a operating system environment?

If I understand you correctly, it will do both.

will this adapter work with a macbook pro 2008 laptop?

In the product description it says is supports "Mac OS X 10.8". I don't know much about Mac's but if your operating system is the same as this then they say it is supported. I bought it for Windows so do not have first hand experience of the Mac.

Questions & Answers: Coboc AD-U22VGA-6-BK 7 inch Black color USB 2.0 to VGA External Video Card Adapter USB to Display Graphics Converter– 1920x1200/1080P

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Coboc AD-U22VGA-6-BK 7 inch Black color USB 2.0 to VGA External Video Card Adapter USB to Display Graphics Converter– 1920x1200/1080P

7 inch USB 2.0 to VGA External Video Card Adapter
Support PC 1920 x 1200 and HDTV 1080p Analog Resolution
Support Mirrored,Extended or Primary display configurations

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