Patches that you apply do not overwrite the OEM customizations.
http://www.vmware.com/files/pdf/ESXi_management.pdf :
The core hypervisor code also can contain custom code provided by server vendors (OEMs) that provides additional functionality, such as hardware monitoring and support information. These customizations would be present, for example, if ESXi had been obtained in embedded form from the server manufacturer or if a custom version of ESXi was installed onto
the hard drive. Any update to an existing ESXi installation automatically incorporates the proper update to this custom code.
Dave
VMware Communities User Moderator
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