Yes, you are right. I spent a whole day try to figure out how to enable VT in the ESX guest VM but no luck.
The final result it needed to upgrade ESX 4.1 to 5.1 and added vhv.enable = true on the ESX 5.1 physical host's config file.
Afterward no more problem with the ESX 5.1 guest VM with 64 bits O/S guest on guest.
Conclusion: ESXI or ESX 4.1 does not support nested ESXI for 64 bits O/S. It will work fine for 32 bits guest on guest O/S.