HyperThreading is here only to help you offload some work simultaneously with one compute cycle - the performance gains are 20% per hyperthreaded core at best and only with certain workloads on desktop, I'm not even sure that virtualization can yield that much performance increse. You always associate a physical core to a vCPU and HT steps in where help is needed - you can observe esxtop while the VM is running - great guide is here ESXTOP - Yellow Bricks and press "C" for monitoring how the workload jumps around the physical CPUs - the VM does not stay only on one core - the VMkernel schedules the virtual machines to run on the least contended CPU.
My best bet would be that HT helps offload 5-10% of the total CPU frequency, but I'm really just shooting in the dark here ![]()