I see Update 2 is available for ESXi 5.1. We are an HP shop. Hosts were updated from 5.0 to 5.1 using the HP custom iso.
Should I be using the HP update 2 iso rather than the patches to update these servers?
I think you should add the HP repository into update manager, and then create a host extension baseline that includes the currently available HP-specific updates.
See particularly the "HP Online Depot in the Software Delivery Repository (SDR)" in this document:
And see "customizing VUM for your HP servers" from this document: