The latest VMware ESXi hypervisors don't use emulation, they use hardware-assisted virtualization, taking advantage of the extensions in the Intel VT-x and AMD-V technologies.
The VMware ESXi hypervisor time-slices the physical processors across all virtual machines so that each virtual machine gets a piece of the action. This abstraction is what allows you to assign virtual CPUs to virtual machines. If multiple virtual machines are running, the ESXi hypervisor allocates a share or slice of the physical processors to each virtual machine. If the default resource allocations are used, then all virtual machines get an equal share of resources.
When available resource capacity does not meet the demand of the virtualization overhead and virtual machines, customization of resource allocations is possible in ESXi through the vSphere Web Client.
If a virtual machine is assigned twice as many shares of a resource, such as a vCPU, than another virtual...