It seems as if the recipient is not set correctly in
Code:
maildrop unix - n n - - pipe
flags=DRhu user=vmail argv=/usr/local/maildrop/bin/maildrop -d ${user}@${nexthop} ${extension} ${recipient} ${user} ${nexthop} ${sender}
in master.cf. What happens if you replace ${recipient} with a hard-coded email address (e.g. yours) for testing purposes? Do you receive the autoreply emails then?