Once a vCloud instance is setup, you will have the ability to add "Organizations" - these objects have users accounts, virtual datacenters (where you setup what storage/compute/network they have access to). Once these objects are configured you would give a URL to the new admins.
normally: https://yourcloud.yourcompany.com/cloud/org/TheExternalCompany
From that URL with the user-account created they would be able to connect and do the required functions.
For the VM's and what IPs they will get - when you setup the "OrgVDC Network" (one of the resource constructs inside the Virtual DataCenter) you will be able to set IP address pools ect.
Hope this gets you started - really with vCloud you have to get an instance up and start playing with it some to get a good handle on the concepts.