As far as I can see this is impossible to emulate with --virtual-config-dir, as it only specifies the DIRECTORY and expects a file user_prefs, with no way to specify an alternative filename. This would mean I'd have to have something like:
/var/spool/userprefs/username/user_prefs
Read the spamd man page more closely.
http://spamassassin.apache.org/full/3.0.x/dist/doc/spamd.html
You can specify variables in the path. It's quite flexible.
You probably want:
--virtual-config-dir=/var/spool/userprefs/%l/
