I have managed to "hack" (sorry) the language file, it is located in the administrator/components/com_vminvoice/helpers folder and is the vminvoice.php file.
Simply edit the JText::_('VMI_XXX' area inbetween the ' '
For example
JText::_('VMI_DELIVERY_NOTE') to JText::_('something')
and it will change.
You indeed can also use the language file especially if your multiple languaged.
I have also managed to insert a rule...
Thats the total amount excluding total tax. (something thats mandatory in dutch invoices)
If someone is dutch and doesnt know how to edit this, you can email me so i can send u the file.