And why would you like to add cores to a virtual machines vCPU´s? well as the following table shows, if you have a physical win 2008 r2 standard with 2 pCPU´s and 4 cores each and you want to virtualize the machine with the same amount of cores for processing, the virtual machine cannot utilize the 8 vCPU´s because of the version of the operative system.
How do we in the virtualization platform then?
In VMware vSphere you can use the advanced settings and configure cores on vCPU and lure the operating system to use 8 cores..
there is a VMware KB for this 1010184
If you set up a new virtual machine you would probably set it up with enterprise or datacenter edition and then this is not an issue. there might be software that is licensed per cpu and then this could be a way to use more cores instead of more vCPU´s and get better speed and still be in compliance with the license.