Virtual machines require a certain amount of available overhead memory to power on. You should be aware of the amount of this overhead.
The amount of overhead memory needed for a virtual machine depends on a large number of factors, including the number of vCPUs and memory size, the number and types of devices, the execution mode that the monitor is using and the hardware version of the virtual machine. The version of vSphere you are using can also effect the amount of memory needed. VMX automatically calculates the amount of overhead memory needed for a virtual machine.
In order to find out how much overhead memory is needed for your specific configuration, first power on the virtual machine in question. Look in the vmware.log file. When the virtual machine powers on, the amount of overhead memory it needs is printed to the log. Search within the log for VMMEM to see the initial and precise amount of overhead memory reserved for the virtual machine.