Hello,
Can you please activate temporally "debug mode" in your Joomla global configuration ?
It will allow you to have the full "call stack" of the error, with the indication of the file where the problem is.
The function getVendorIdFromOrderProducts is indeed a private function and is just used in the HikaMarket order class.
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.