You might want to update the source.list . I am not sure, but I think its not the latest one.
http://software.virtualmin.com/vm/6/gpl/apt/And btw, it wont update to 6.09 by running apt. You manually need to install that.
Thats the link: http://www.webmin.com/vdownload.html
Download it with wget and install it with dpkg -i webmin-virtual-server_6.09.gpl_all.deb