'vadddomain -O' is broken in 5.4.0-pre1, and probably others.
The problem is that vget_assign() looks in [/var/qmail/]users/cdb for the domain, and it's not there yet for the vadduser(postmaster) call.
vsetuserquota(), after vadduser(), also uses vget_assign(), as does vadddomain's main() directly if a catchall has been chosen.
It's not a big deal to me, but I thought I'd point it out because it probably shouldn't be broken in 5.4.0.
Submit a patch on sourceforge or put it on the list if possible, as you know why the problem happens.
If not, let me know where to look exactly, and ill take a look.