You can modify the number of cores per socket like this:

$spec = New-Object -Type VMware.Vim.VirtualMachineConfigSpec -Property @{"NumCoresPerSocket" = 2;"numCPUs" = 4};

$vm.ExtensionData.ReconfigVM_Task($spec);

With this the VM will be configured with a total 4 vCPUs, spread across 2 sockets with 2 cores each.

Also see:

http://communities.vmware.com/message/1891250#1891250