How can I create 2 subsites (at the same level) without one being aware of the other?

I have a site collection called Vendors. I need to create a sub-site for each of my vendors, so I do so by creating a new sub-site with unique permissions.

The problem is that when users of the 'vendor1' sub-site clicks on

Site Settings -> People and Groups, --> More

He is able to see

- vendor1 members
- vendor1 visitors
- vendor1 owners
- vendor2 members
- vendor2 visitors
- vendor2 owners

How can I make it so that users of vendor1 can only see groups of the vendor1 sub-site and users of vendor2 can only see groups of the vendor2 subsite?

Here is my site structure

                           vendors (Site collection)
                        /          \
                       /            \
                      /              \
                   vendor1          vendor2