Hi,
In the folder "plugins/hikashop/attachinvoice/attachinvoice/" you would found the file "invoice.php".
You have to copy this file into "media/com_hikashop/plugins/invoice.php".
After that, you would be able to modify this new copied file in order to override the original one (like view overrides in HikaShop).
So, you can add some information like bankaccount and VAT number.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.