VMware FT used on VMs running on nested ESXi hosts is unpredictable -- sometimes it works and sometimes it won't work.
Here are comments and suggestions:
Make sure the VMs you're trying to use with FT have only one vCPU assigned.
I'm assuming your nested ESXi hosts are also ESXi 5.1 just like your physical ESXi 5.1 host. If not, stick with ESXi 5.1 only and make sure it's ESXi 5.1a or higher.
1) You must have EPT (Extended Page Tables) working for Nested FT to function -- the quickest way to verify this is functioning is to install any 64 bit VM onto the nested ESXi 5.1 VMs and verify the 64 bit VM is working properly. If that fails, come back here and ask about how to get that working. For now I'll assume you already have EPT working.
2) Read the instructions and following these instructions at this link:
http://www.virtuallyghetto.com/2011/07/how-to-enable-nested-vft-virtual-fault.html
and, as described in that link, add the following to your configuration of the VM you're trying to use FT on (don't add this to the nested ESXi VM):
replay.supported = "true"
replay.allowFT = "true"
replay.allowBTOnly = "true"
Also, then add the changes relating to Migrate/VMotionStreamDisable as described in that article in the link above.
Again -- you might or might not be able to get this working in a nested situation. It's a roll of the dice. Sometimes Nested FT will work and sometimes it doesn't. Since you're using this for a training lab, you may just be able to get the secondary VM tostart up enough to simulate the learning necessary for studying purposes but not fully get VMware FT fully functional.
Datto