Always upgrade vCenter Server to version 5.5 Update 3b before you update ESXi to ESXi 5.5 Update 3b or later to avoid issues due to interoperability implication relating to SSLv3 disablement.
vCenter Server will not be able to manage ESXi 5.5 Update 3b or later versions if you update the ESXi hosts before updating vCenter Server to version 5.5 Update 3b. VMware best practices states that the vCenter Server should be upgraded prior to upgrading the managed ESXi hosts. For more information