Let me see if I understand correctly. You are trying to create a movie with a colored vignette (either white or black), kind of as a frame for your video?
What format is the video you are importing? You say it is a .BMP? I am not aware of .bmp files being able to contain video or transparencies. I assume that you are creating a movie by importing a series of still .bmp files, and Corel is saving them as .avi - does this sound correct?
Assuming the above is true, one thing to keep in mind with movies is that if you don't "combine" an image with the background for a particular frame, it will "float" above all the frames. So if you have an image or mask, and are applying a vignette to a portion of that image, then not combining that image with the background frame, you will loose you transformations when going to the next frame. This could explain why some of your vignettes are missing or different colors. Case in point, if you touch up a photo, or clone a small section somewhere else, and don't combine it with the backround, you will see that same transformation in all frames. This is ok if the transformation is out of place, you will catch your mistake. However, if your transformation applies to all the frames, then you will never know that only one of the frames has been transformed, leaving the others in their original, un-transformed condition. This is probably what is happenening with your vignette. It is proably floating on a potion of an image on the first frame, and is being caried through all of your frames. Combine your first image with the background and see if the second frame is mising the vignette. This should make my explanation a little clearer. You can soon start to see the frames that have been combined, and the ones that haven't.
So the best thing I have found is to set all of your pictures and transformations in the appropriate frames first. Go through and make all of your modifications to each frame. When you can go through the movie, and see the images and transformations as you want them, then make the last step to apply you vignettes, after each of the images are combined with the backgrounds.
As for image/quality loss, it probably occurs when saving your image as a .gif (I assume for web display). Check out thread240-323232 as it goes into some detail about 8 bit images. Just to let you know, I found the best results are using uniforme pallete with ordered dithering (for photos). THe preview may look a little grainy, but the final output is quite acceptable (depending on video size). It is never as sharp as the originals, but color stays rendered properly, and you don't get a lot of "jaggies", just an exceptable bit of loss of resolution (assuming your resolution is high enough to begin with).
HTH,
Russell