why won't a Vivitar DVR camcorder work

It would have speeded up a reply if you had bothered to mention what version of iMovie, on what Mac, running what version of OS X.

iMovie 11 - cameras supported:

http://help.apple.com/imovie/cameras/en/index.html?lang=en_US

iMovie 9 - cameras supported:

http://support.apple.com/kb/HT3290?viewlocale=en_US

iMovie 8 Camcorders supported:

http://support.apple.com/kb/HT1014

Digital camera RAW formats supported by OS X Lion:

http://support.apple.com/kb/HT4757?viewlocale=en_US

Digital camera RAW formats supported by OS X Snow Leopard:

http://support.apple.com/kb/HT3825?viewlocale=en_US