It's not necessary to boot from a LiveCD or linux rescue disc.
When the appliance starts, the GRUB boot loader shows briefly.
- When it appears, hit the up or down arrow to stop the boot countdown clock (which is only like 2 seconds).
- Following the instructions at the bottom of the screen, hit "p" to enter the unlock password (which is the root password). This will allow you to change the boot-time string.
- Highlight "VMWare vCenter Server Appliance" and press "e" to edit the boot settings.
- Highlight the "Kernel..." line and press "e" to edit the boot string.
- Append a "1" to the end so that it looks like this: "...showopts 1" (minus the quotation marks)
- Press enter, and then "b" to boot.
This will cause the appliance to boot in init level 1 (or single-user maintenance mode). From here, the root password will get you into the console where you can delete /etc/vmware-vpx/ssl/allow_regeneration. Reboot the server when you're done with "shutdown -r now". It shouldn't be necessary to go back into GRUB to remove the "1" you added earlier.