We just made a significant improvement to the way vacation messages work. In the old system, if you turned on a vacation message, your mail filters would stop working. Now, turning on a vacation message will not stop your mail filtering rules from applying. This includes any mail forwarding rules you have setup. Of course you still need to leave a copy of the message on the server when setting up mail forwarding. In order for these changes to take affect, you will need to turn off your current vacation message and then turn it back on. And remember, only one vacation message is sent to each sender no matter how many times they send email to the person with the vacation message set, until the vacation message is reset.
Thanks to Kevin for knocking this out, based on customer feedback, in his spare time.