Hi,
If the vendor creates the product in the front-end, the assignation will be automatic.
If you want to create (or edit) a product in the back-end, there is a field which allow you to select the vendor ; just under "published" (named "vendor").
For the category, you can't really assign a category to a vendor, in the way that a vendor can have a "root category" but it can't own several categories.
If you set the option "Change vendor root category" to yes, you will be able to choose the root category for each vendor.
Or you can use the options "create categories for vendor" and "Change vendor root category = vendor home" in order to have something automatic.
Regards,
PS: For your HikaMarket questions, please use the HikaMarket forum section, otherwise I won't be triggered that there is questions for HikaMarket.