This topic might be a little old but I had a very similar problem recently - Vsphere Client (5.0) refused to launch, just a meaningless "VpxClient has stopped working" message pops up briefly with no further explanation.
I eventually found that a Windows Update (KB2729452) seemed to be the cause, having broken mscorlib.dll
I followed the instructions at http://support.microsoft.com/kb/929833 to find the culprit and when I removed the update (twice!) it all started working again.
(all attempts to replace the mscorlib.dll had no effect but uninstalling the update did the trick)
Hope this helps someone else finding this thread.