Lighting the screen properly is the key to chromakey. Green is the preferred because most DV cameras use green for the luma information and you get fewer aliasing artificts (jaggies). Green is also the least likely color you will encounter in your subjects, but with Vegas you can use any color that is not in your foreground subjects.
Light the screen evenly and keep your subjects as far from the screen as practical. Do not make the screen so brightly lit that the subjects are being illuminated from behind by the light reflected from the screen.
Steve Mann