Demo http://demo.plusmotiv.com/vendorstarter/admin admin/admin
Create vendors from admin part with username and password
Check orders for each vendor in admin part
Vendors can log in with that password in www.yoursitename.com/vendor
They can enter ,edit ,delete products that are only for them
They can see orders with shipping address,payment address,shipping and payment method and their products.(without totals and shipping cost)
Separate folder for each vendor.
Separete workspace for each vendor.
All products and orders from all vendors are visible in admin
In vendor workspace user can see only his products and orders
Check more here http://demo.plusmotiv.com/vendorstarter/admin admin/admin
After you create vendor make sure that he is enabled and go to http://demo.plusmotiv.com/vendorstarter/vendor and log in.
Update I added modules for
display vendors on fronend, you can download for free http://www.opencart.com/index.php?route=extension/extension/info&extension_id=20656
and for displaying vendorname in admin products list ,also free http://www.opencart.com/index.php?route=extension/extension/info&extension_id=20657
for opencart 2.0+ see this module www.opencart.com/index.php?route=extension/extension/info&extension_id=22824