Dovecot LDA with Qmail
System users
The delivery command you need is
|/var/qmail/bin/preline -f /usr/local/libexec/dovecot/deliver
(You may need to adjust the paths to match your qmail and dovecot installations.) The preline command will add the Return-Path: and Delivered-To: lines, because deliver doesn't recognize qmail's environment variables.
For site-wide usage, put that in /var/qmail/control/defaultdelivery (assuming you installed qmail according to LWQ). Or, save it as .qmail in selected users' home directories.
Virtual users
Add the -d parameter to specify the destination username:
|/var/qmail/bin/preline -f /usr/local/libexec/dovecot/deliver -d $EXT@$USER