Question on categories when adding new vendor

-- HikaShop version -- : 2.3.3 business
-- HikaMarket version -- : 1.4.4
-- Joomla version -- : 3.3.6
-- PHP version -- : 5.4.33

I"m sorry but the documentation is vast, and a bit confusing.

Here's what i want to do:

I have Hikashop Business and HikaMarket. latest versions on Joomla 3.3.6

We have 400+ skus already in our store from a single vendor. We have a number of product categories, ie, Men's Shirts, Women's Jackets etc. We imported and set up from an old Virtue Mart shop.

We now want to add a second vendor. And more vendors in future. So i purchased HikaMarket.

I want this to be invisible to the user. They won't know that there are multiple vendors. I want a vendor to be able to add and edit their products in EXISTING categories, that is, Vendor1 and Vendor2 can both put products into Men's Shirts.

But not see the other vendor products.. can't edit stock quantities, can't delete. A vendor should be able to edit only his own products.. but all vendors have stuff in same categories.

How to do this? I"m confused about vendor categories. What do i need to do to all 400 existing products before adding new ones from Vendor2?

THank you for that quick answer, very much appreciated.

If i might ask one more key question: as we already 400 + products.. what is the best way to assign all existing products to one vendor... before we start adding new products for our second vendor.

Is there a batch process avaiable in HikaMarket to do so? I saw some discussion about that but it appeared that was still in development. How best to do this? thanks