Hyper-V does not allow to be installed on a machine that's already virtual (such as on another Hyper-V or VMWare). For VMWare, you can circumvent this by adding custom configuration parameters to the VM's configuration.
Add these to the VM's configuration, either through the vSphere (web)client, or by directly editing the VMX-file:
hvh.enable = TRUE hypervisor.cpuid.v0 = FALSE
If you're editing the VMX-file, you need to enclose the values (behind the =) in double quotes (").
Shutdown the VM.
Open .vmx file with Notepad or another plain text editor.
Add these lines:
hypervisor.cpuid.v0 = "FALSE" mce.enable = "TRUE" vhv.enable = "TRUE" vpmc.enable = "TRUE"
Save and start the VM. Now you can install the Hyper-V role.