First of all, I apologise for the double post.
I have check the apache2.conf, it does include the Vhosts_ispconfig.conf at the end.
However, when I check the contents of the file Vhosts_ispconfig.conf, it shows an empty file. However the others files (i.e. Vhosts_ispconfig.conf_DATE_TIME) in the folder vhosts have contents.
It is so strange. Any hints?