Hello,
If you remove the ACL for the category publish ; your vendors would be able to create new categories but won't be able to change that state. And, in such case, the category will be unpublish by default.
You will have the control for the validation (and publishing) of the category but you will be sure that you keep a control on the category creation.
Otherwise, I would suggest a view override if you want to change the default state ; in PHP you can detect if you're creating a category or not (because you have a category_id, or not).
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.