“Timed waiting for vpxa to start” is an error message you may receive when trying to add a VMware vSphere 5.1 ESXi host to a vSphere vCenter Server 5.0/5.0 U1 environment.
There are a couple of reasons for this particular error message:
1. One or more of the VMs on the ESXi host are running with a large number of snapshots. Reminder: the maximum is 32. See this VMware Knowledge Base for more details.
2. The second possible reason for the error message is that ESXi 5.1 hosts can only connect to VMware vCenter Server 5.1 instances, this can be confirmed in the VMware Product Interoperability Matrixes which can be found here. As you can see, from the matrix a VMware ESXi 5.1 host is not compatible with anything but VMware vCenter Server 5.1. So, if you want to add your ESXi 5.1 host(s) into a vCenter Server instance then you can either plan to upgrade your vCenter Server instance to 5.1 or downgrade to the ESXi host to 5.0 U1 or earlier.
Either way bookmark the VMware Product Interoperability Matrixes page as it is a useful resource and can remove the need for any VMware versioning compatibility guesswork.
Also, if you are only dealing with ESXi 5.0 and vCenter Server 5.0 (ie: no ESXi 5.1 host(s) involved) and are experiencing this error then take a look at this VMware Knowledge Base article here.