Can you just add some folders in VirtualCenter, add users to the VC machine, and apply permissions that way? Something like:
Local Users - companyAuser, companyBuser
Local Groups - companyAadmins, companyBadmins
Add folders under your Data Center - CompanyA and CompanyB
Put the appropriate VM's where they belong.
Right-click on the CompanyA folder and "Add Permission" - Add the companyAadmins gropu, and assign the Administrators role to that group. Repeat for CompanyB.