The aspect ratio of BMP images extracted from PAL VOB files on DVD have the dimensions 720 x 576 which is NOT the 4:3 VGA ratio required.
For a BMP with the dimensions 720 x 576 all that is required is to rescale (not maintaining aspect ratio) to 640 x 480, or to the larger PAL 4:3 image size of 768 x 576
Rescaling should be done at the end when the final image is obtained from Registax or Deep Sky Stacker.
Steve Wainwright