Support from VMware is pretty good. I submitted the support ticket after having encountered two "purple screens", and got reply almost immediately the same day with the request asking me to run a built-in tool called VMware-support on ESX server to collect all log information first.
Since ESX is based on Red Hat Linux, I had to run the diagnose tool right on the server and used WinSCP to download the result file to my laptop and uploaded it to their FTP server. While I was waiting for the news from VMware, I noticed that there were some warning messages popping up during the boot process telling me something about the BIOS, "machine type mismatched" more accurate. I thought it might be the reason causing the "purple screen", so I just went ahead and updated the BIOS for the server, and the server was running fine the rest of day without any further "purple screen". Next day, I got the answer from VMware that proved my thought as the log file I collected specifically pointed that there were hardware incompatible issue on the server.
It turns out, the purple screen shouldn't happen at all if the hardware are all compatible with ESX. Since the ESX server is heavily relied on the hardware layer, it would be good and healthy 1) make sure all hardware are compatible; 2) firmware for the hardware needs to be up-to-date.
Another lesson has to be learned and remembered.