I am copying this message to vufind-tech and vufind-general so that it will reach a larger audience; vufind-admins is largely used for automatic notifications.
I don't know specific details of Koha, but I'm sure somebody can tell you how to export MARC records - given that this is an open source project, I am sure there are good tools available for dealing with exports.
Once you have the Koha-specific details, automating updates is simply a matter of creating a cron job that exports new records on a regular basis and then runs VuFind's import-marc.sh script. You may also want to run the suppressed.php and deletes.php utilities to deal with suppressed and deleted records if this is applicable for Koha. It is also useful to run the optimize.php script after updates to ensure that your spellcheck index is always kept up to date. (All of these tools are found in the util folder of VuFind).
It would be nice if somebody could update the export page of the wiki with some Koha details for future reference - I'm sure others would find it useful:
You may also find it useful to look at the automation page for some further tips - there is an example there showing how VuFind can be automated with Voyager; the exact details for Koha will be different, but some of the processes may be similar:
From: Arup Raychaudhury [mailto:arup.raychaudhury@...]
Sent: Wednesday, September 18, 2013 5:57 AM
Subject: [Vufind-admins] How to get Koha data into VUFIND
I have installed successfully vufind2 on Ubuntu 12.04. Same machine has KOHA 3.12. During installation of vufind, I have selected KOHA Connector and have made necessary changes in Koha.ini.
Can you explain how to get MARC21 data from KOHA to vufind. Can I periodically update the data transfer without human intervention?