When migrating product data from VirtueMart to Magento, there might be some things that you need to change: Some attributes change name when migrating to Magento, or some attribute names are simply wrong. Vm2Mage allows for a custom plugin to change the migration specifically for your project.
Changing data
Whenever an item (user, product, category) is migrated from VirtueMart to Magento, the following 2 events are triggered by Vm2Mage:
onVm2MageItemConvertBefore($item, $type)
onVm2MageItemConvertAfter($item, $type)
These events allow you to write a plugin that changes the data before they are sent to Magento. The $item variable is a PHP reference: If you change the variable within your own plugin method, you will change it in the original data-set as well. It is an array containing all relevant data. The $type variable is a string, being either user, product or category.
The onVm2MageItemConvertBefore event allows you to manipulate the raw VirtueMart data as they are loaded from the database. Fields often contain a VM prefix. With the onVm2MageItemConvertAfter event, those raw VirtueMart data have already been manipulated a bit by Vm2Mage.
Changing attribute names
One other useful event is the event onVm2MageAttributeLoadName:
onVm2MageAttributeLoadName(&$attributeName)
Within VirtueMart, the requirements for attribute-names are really loose: Attributes can be lengthy. They can use normal western characters or non-western characters (like Arab, Sanskrit or Thai). WIth Magento, the requirements are very strict: Attributes can only have a name with alphanumeric characters or underscores. Vm2Mage normally already applies a few tricks to remove all illegal characters, but this might cripple your attribute-name. This event allows you to change attributes before they are being migrated by Vm2Mage. Vm2Mage also removes the com_virtuemart_ prefix from attributes.
Example plugin
An example plugin can be found here:
https://gist.github.com/yireo/d937cb17126d542a69c2
Debugging
To debug the resulting data, open up a product within the Vm2Mage backend in Joomla and hit the Debug link. This will show you a dump of all the data contained within a product. It will allow you to see whether your plugin had any effect or not.
About the author

Jisse Reitsma is the founder of Yireo, extension developer, developer trainer and 3x Magento Master. His passion is for technology and open source. And he loves talking as well.