In the VMD installation directory ($VMDDIR/scripts/vmd/) there is a perl script called chemical2vmd which will create a VMD command file. Since
then start VMD with the -e command line option to read that file. The file contains the necessary commands to convert
It is also possible to install the previous script in the global .mailcap file to make it accessible to everyone. You will have to consult the documentation for your web browser(s) to find out how.