You can only use one virtual CPU without VT-x. Thus, the error message stating that VT-x isn’t enabled is caused by the following line:
vb.customize ["modifyvm", :id, "--cpus", "2"]
Replacing it with the following works (Vagrant has a shorthand for setting the CPU count):
vb.cpus = 1
NOTE: You can only run 32-bit VMs inside another VM.