I made a patch which enables you to specify an interval between vacation notifications.
If the interval is specified as 0, everything works as without the patch (only one notification per sender/recipient).
I'm still not sure what to do in the case of errors: send the vacation message anyway, or do nothing.
I would like if someone could review the code.
The patch is against vacation.pl found in Postfix Admin v.184.108.40.206