The feature was introduced to provide better support for applications (and operating systems) that treat cores differently to processors, this is generally due to licensing constraints. XP Home, for example, will not recognize a second processor but has no problem with a single multiple core CPU, likewise Windows Server 2003 Standard has a 4 processor limit but you could configure 4x2 core vCPU's if you wanted to give such a VM the maximum amount of CPU resources possible.
There is no performance difference between vCPU's whether they are presented as separate processors or as multiple cores. A word of caution is advisable though as you seem to be planning to configure a VM with a large number of vCPU's. Performance for multi-vCPU VM's is a tricky balancing act. The VM scheduler has a harder time scheduling multiple cores\cpu's and the more you throw at a VM the harder that gets. So while adding vCPU's should give the VM more processor resources over time it may also result in much more erratic scheduling since the kernel has to wait for all the required cores to be available at the same time in order to schedule any processor resources for the VM.