So I am aware that VMs in FT mode are limited to a single vCPU. Is it possible to assign multiple cores to the single vCPU, and still have the VM support FT mode? Or are you forced to use both a vCPU and single vCore per FT VM?
No, you cannot. VMware only supports a single core on a FT VM. This site has a good reference.
That said, VMware FT has a whole host of limitations. You'd be better off utilizing your application/OS failover methods.