Same for me!
Once an invoice is generated, you cant display it...
I have this message: "Fatal error: func_get_args(): Can't be used as a function parameter in /web/htdocs/www...../home/administrator/components/com_vminvoice/helpers/invoicehtmlparent.php on line 49

I use Joomla 1.5.21
VM 1.1.8
vm invoice 2.0.20