A virtual machine (VM) does not keep time exactly the same way that a physical machine does. When running low latency, high throughput applications like ESP, these differences can cause timekeeping inaccuracies and other problems.
There are configuration options that can mitigate these problems, such as configuring the VM to a particular set of CPUs, or using locked memory (no sharing between VMs). However, these options involve trade-offs with the reasons for using VMs in the first place, such as better resource utilization.
For a detailed discussion of timekeeping in VMware virtual machines, refer to http://www.vmware.com/files/pdf/Timekeeping-In-VirtualMachines.pdf.
If you want to run ESP on a VM, we highly recommend that you engage the vendor for help in configuring the VM for use with ESP.