Compare your understanding with memory diagram at VMware KB 2017642.
Now another question. Do you still think you are able to exactly know how much memory is used and how much is available? Do you? It is very important to know that this task is complex in any operating system because of lot of memory virtualization layers, memory garbage algorithm, caching, buffering, etc .... therefore nobody is able to know exact numbers. Of course you can monitor ESX memory usage but that is always estimated number.
Real memory over allocation and potential memory issue can be monitored by several mechanisms
- Running VMs ballooning - because ballooning starts only when there is not enough memory
- VMs (ESX) swapping - mainly swap in/out rate higher then 0 because that's the real indication you have memory problem