Nice to see that you are using my plugin for the customer numbers.
Unfortunately, according to some bug reports I got, it appears that VMInvoice and my Ordernumber plugin do not work well together to set the order and invoice numbers:
www.open-tools.net/forum/ordernumber-plu...pdf-invoice.html#417
As I don't have a copy of VMInvoice and the developers didn't respond to my requests so far, I can't really find out how VMInvoice assigns order/invoice numbers and how to make our two plugins work together nicely (i.e. use VMInvoice for the invoice handling, but use my plugin to create the order/invoice numbers).
So, if any VMInvoice developer reads this, could you please get in touch with me (office@open-tools.net), so we can make sure our VirtueMart extensions work together nicely.