keep a look at the cpu ready value of your vCPU's. Once it reaches a threshold of 10% (2000ms in vCenter performance chart - real-time) your vCPU to pCPU consolidation rate is far too high.
Of course it all depends on your requirements in your applications / SLAs.
ready is a good indicator, since it reflects the "processing-latency" of your virtual machine. There are a lot of articles explaining this