hostd is an app that runs in the Service Console that is responsible for managing most of the operations on the ESX machine. It knows about all the VMs that are registered on that host, the luns/vmfs volumes visible by the host, what the VMs are doing, etc. Most all commands or operations come down from VC through it. ie, powering on a VM, VM vMotion, VM creation, etc.
vpxa also runs on the Service Console and talks to VC. I believe it acts as an intermediary between VC and hostd. I think it also does some housekeeping on the ESX host, but not as much as hostd.