Because make-special-vdr.sh needs to modify debian/rules itself. This way debian/rules doesn't get "contaminated" with stuff that goes beyond the scope of building the regular Debian package -e except for the shebang line.
Why not so it the other way round, i.e. start two different scripts (or the same script with different parameters) from a debian/rules Makefile depending on the environment variable?
-- Dipl.-Phys. Fabian Greffrath Ruhr-Universität Bochum Lehrstuhl für Energieanlagen und Energieprozesstechnik (LEAT) Universitätsstr. 150, IB 3/134 D-44780 Bochum Telefon: +49 (0)234 / 32-26334 Fax: +49 (0)234 / 32-14227 E-Mail: greffrath@leat.ruhr-uni-bochum.de