On my system everything works just fine
# uname -a
Linux myhost 2.6.18-53.1.19.el5.028stab053.14PAE #1 SMP Thu May 8 21:09:32 MSD 2008 i686 i686 i386 GNU/Linux
# lsmod | grep vzmon
# modprobe vzmon
# lsmod | grep vzmon
vzmon 46984 0
vzdev 7556 1 vzmon
ipv6 262048 21 vzmon
# lsmod | grep conntrack
Moreover, vzmon doesn't depend on ip_conntrack (look at the provided output) so I'm not sure how "ip_conntrack: Unknown parameter `ip_conntrack_ftp'" relates to the problem described. Please, examine dmesg another time and try to find the appropriate message.