Adding the script /etc/mailman/virtual_to_transport
sed -r -e 's/(^[^#]\S+\s+).+$/\1local/' $1 > /var/lib/mailman/data/transport-mailman
When you run bin/genaliases mailman will generate the transport-mailman with a list aliases mapping to local. Example,
# STANZA START: test
# CREATED: Sun Feb 3 12:49:43 2013
Why this works is beyond my understanding of mailman. If you look at virtual-mailman that mapping should have picked up the mapping and directs the mail to aliases which then delivers to mailman. With the transport-mailman mapping, essentially you are sending it to the local system and letting the local services deal with it. I think.