The best way is to modify quicktables script (most probably, it is rc.firewall in your virtual servers). Just add `venet' in list of the recognized devices.
Something like this:
--- /usr/local/sbin/rc.firewall.ovz 2003-09-02 08:04:54.000000000 +0400
+++ /usr/local/sbin/rc.firewall 2006-12-06 12:08:45.000000000 +0300
@@ -53,7 +53,7 @@ date=`date '+%Y.%m.%d.%S'`
### define regex for ip and interface validation ###
is_ip="grep -Ec '^[1-2]?[0-9]?[0-9]\.[0-2]?[0-9]?[0-9]\.[0-2]?[0-9]?[0-9]\.[0-2]?[0-9]?[0-9](\/[0-3]?[0-9])?$'"
-is_if="grep -Ec '^(eth|ppp|wlan|tun)[0-9]$'"
+is_if="grep -Ec '^(eth|venet|ppp|wlan|tun)[0-9]$'"
### don't overwrite existing output file if present ###
if [ -f $out ]; then