I've had mixed results with getting vFlash metrics also, some VM's show metrics straight away, some after a couple of hours, some never show.
In ESXi the /var/log/hostd.log there will be messages similar to below indicating a problem.
2014-12-18T02:20:40.023Z [3DF42B70 error 'Statssvc.vim.PerformanceManager'] CollectVmVdiskStats : Failed to get VFlash Cache stats for vscsi id scsi0:1 for vm 16
When I raised a support request on the issue, VMware Support advised they have seen this problem with a few other customers and there is a bug raised with VMware engineering. If you experience this problem contact support and provide them with logs to help track down the cause.
I'll post updates to my support request and hopefully the solution.