Setting up vaction with procmail

First you need to add a vaction entry to procmail. There can only be one of these, if you already have one you'll need to delete it first.

You'll want this entry to appear after any spam filters, but before any other rules. You'll also want to select the "Use a copy of the message for this entry" checkbox or your mail will be responded to then thrown away (You'll be warned about this if you forget).

You will then be presented with a box to enter your vaction message in:

This is the message that will be sent to anyone sending you mail while this rule is in place. This message will only be sent in response to messages sent to one of the aliases listed in the aliases box. Each person sending you a message will only get this response every "Only send autoreplies every" days.

This will generate an entry that looks like this: