[Mimedefang] vbounce when using mimedefang & spamassassin

[John Mathews]

Recently installed from source mimdefang (mimedefang-2.64) & 
SpamAssassin version 3.2.4 running on Perl version 5.8.8. 
I had some trouble understanding what changes I needed to make in 
/etc/mail/sa-mimedefang.cf but have most things I need up and running.
The machine runs sendmail and acts as a hub only, never any local users.
I can't seem to get the vbounce ruleset to function under mimedefang.  
I've placed the recommended lines into sa-mimedefang.cf:

whitelist_bounce_relays myrelay.mydomain.net

I can test the rule by
spamassassin -Lt < sample-vbounce.txt and it works OK.

I don't know how to do a corresponding test which uses mimedefang to 
invoke spamassassin; calling spamassassin from the shell reads 
configuration from /etc/mail/spamassassin/local.cf and not from the 
sa-mimedefang.cf file.
My question is why doesn't the vbounce ruleset  fire when mimedefang 
invokes spamassassin, and how can I test what the changes I've put into 
sa-mimedefang.cf are doing?
thanks,
John Mathews