You can customize OS distributive URLs in a separate VPS config file located in the $VPSCONFIGS/$VPS_DISTR_URLS (/hsphere/local/config/vserver/vps.distr.url by default).
File format example:
CentOS3="ftp://ftp.arcticnetwork.ca/pub/centos/3/os/i386/RedHat/RPMS
ftp://ftp.arcticnetwork.ca/pub/centos/3/updates/i386"
RH73="ftp://fr2.rpmfind.net/linux/redhat/7.3/en/os/i386/RedHat/RPMS
ftp://fr2.rpmfind.net/linux/redhat/updates/7.3/en/os/i386"
WBEL3="ftp://ftp.esat.net/mirrors/whiteboxlinux.org/3.0/en/os/i386/RedHat/RPMS
ftp://ftp.esat.net/mirrors/whiteboxlinux.org/3.0/en/updates/"