I'm having this strange issue on a clean install of ESXi 5.0 and adding it to vCenter. It times out with the error timed vpxa out. The vpxa logs on the server contain no information as well. I did some digging and tried blowing away the vpxa config and restarting the services with no luck. I restored the config file and added a <hostIp> since that was missing and that did nothing as well.
There is not a firewall between the ESX and vCenter server.