Hello,
When you add a user, you need to click on the button to add the selected user into the list before saving the vendor.
Regarding your SQL error, best would be to activate temporally the joomla debug mode so you will have the full error message including the file where the query is generated and the entire SQL query.
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.