Hi Marek,

I have just done this myself. I have a pretty isolated environment and only 1 VM will have access to internet. Rest are isolated for security reasons. I have had to use SAM

Here are couple of pointers.
1: you need 2 VMS, one to run Subscription Manager and another one that will run virt-who both are Linux boxes
2: your need to register the VMs to the SAM VM not to Redhat Customer Portal
3: Not sure if you have a virtual center or not but you can use that instead of the ESX hosts
4: Once you have Subscript manager (1.3 or higher) installed you need to go to the REdhat Customer portal and in Subscription Management Applications you need to create an organization. To this organization you add ALL your subscriptions for your Hypervisors. Once you have this download the Manifest.
5: In your SAM VM you need to upload the manifest. Pretty straight forward you can do it from the GUI
6: you then need to register the virt-who vm to it. Here is Link that explains how to do this https://fedorahosted.org/katello/wiki/GuideSystemRegistrationClient Note you are NOT yet adding subscriptions yet.. in fact you can't
7: Once your virt-who vm is registered you can start the virt-who service. This will get the UUIDs of your ESX servers and present them to your SAM under systems. Now you can register your Hypervisors.
8: once your hypervisors are registered and have a subscription you can then register VMS to them and assign them subscriptions.

couple issues:
1: the license you have does not provide you with product information so if you want to set up a reposerver its messy
2: if you move your VMs to different licensed server virt-who does not update SAM you need to restart the virt-who service
3: redhat tell you you need Administrative rights if you want to get the UUIDS of your ESX hosts through Vcenter, you don't

hope this helps
good luck