This patch really only applies under certain conditions, when PFAs php-script for some reason can't delete the entry from the vacation_notification table (for example if your running MySQL Replication to a off site email server, and you don't like to setup a master<->master replication in MySQL).
What it does is that it enables so vacation.pl takes into consideration that the vacation_notification table can contain outdated values (i.e. a user has changed his message and/or come back from vacation and gone on vacation again).
It creates a higher amount of sql queries as an conditional query is moved to be an needed sql query, so it should really be used when you only have to.
And it should also be combined with an shell script (perl, php, sh) to clean the vacation_notification table.
Patch applies to SVN rev. 535
My suggestion to maintainers is to place it into contrib, this should NOT go into the vacation.pl as it still only applies under very special circumstances.