Of course I assume when it says "drop the message" it means no reply 
is generated for the message, but the message is still delivered 
properly to the recipient.  I can't imagine how vacation itself would 
drop the message without delivery (that would have to be done by some 
actual vacation program, not the Horde vacation module).

If I understand correctly, this is correct behaviour on the part of 
vacation, per RFC 3834.  If there is a problem, it is with Notes 
rather than vacation.  Per the RFC:

       Responders MUST NOT generate any response for which the
       destination of that response would be a null address (e.g., an
       address for which SMTP MAIL FROM or Return-Path is <>), since the
       response would not be delivered to a useful destination.
       Responders MAY refuse to generate responses for addresses commonly
       used as return addresses by responders - e.g., those with local-
       parts matching "owner-*", "*-request", "MAILER-DAEMON", etc.
       Responders are encouraged to check the destination address for
       validity before generating the response, to avoid generating
       responses that cannot be delivered or are unlikely to be useful.

So, if there is no return path, or there is an empty/null return path, 
we should not generate a reply.  More specifically, the RFC states:

    If the response is to be generated after delivery, and there is no
    Return-Path field in the subject message, there is an implementation
    or configuration error in the SMTP server that delivered the message
    or gatewayed the message outside of SMTP.  A Personal or Group
    responder SHOULD NOT deliver a response to any address other than
    that in the Return-Path field, even if the Return-Path field is
    missing.  It is better to fix the problem with the mail delivery
    system than to rely on heuristics to guess the appropriate
    destination of the response.  Such heuristics have been known to
    cause problems in the past.

So, IMHO, this says fix Notes, and leave vacation as is.

What does this have to do with Horde Vacation?

Find a problem with Lotus Notes mail rules , when a mail forwarded 
from Lotus Notes 6.5 forward agent that will not contain return path , 
the vacation modules will drop that message any idea to figure out 
that problem , Thank You !