I had something similar, after restarting the initialised vCenter Applicance (vCSA) 5.0.1.
VPXD service and all the rest with it (including vCenter) would not activate.
After quite some research, including an upgrade to release 5.1.0b dated 18jan13. I found a possible other cause. When the hostname of the vCenter-IP-adress cannot be resolved, things go bad.
As this is/was a test environment, I used a DHCP assigned server adresses for vCenter which (I know 
) is usually a bad idea for server related operation.
In short, I resolved the by adding the "hostname" to the /etc/hosts file, clearing-out the odbc-database settings and re-activating the web-activation.
- SSH terminal logon to the native console-prompt of vCenter
- Stop the VPXD process via command: service vmware-vpxd stop
- Killing the continuous restart watchdog-process PID using "kill ##" where we found the process PID of using "ps -aef | grep watchdog"
- Edit using "vi" the /etc/hosts file and adding the "hostname" of the vCenter-server to the related & resolved IP adress.
- Resetting the database odbc-settings and clearing the database via: /usr/sbin/vpxd_odbcconfig
- Via the web, re-initialise the settings using the "Launch Setup" wizarrd ( http://vcenterip:5480 )
Hope this helps.
I've done quite some investigation on tracing internal (log)files, processes etc.etc and found the above a working procedure that permanently resolved my problem.
Kind regards,
Peter Ooms aka. POCS