As jpsider mentioned, VMware tools aren't REQUIRED for VMs to function. However, for things like vMotion, HA, etc it's much better if the tools is installed. I have some VMs in me environment that purposely do not have the tools installed and I've installed the VMSCSI and VMXNET3 drivers manually.
As you said the tools are installed but not working, you may want to make sure the service is running. This forum post may help.
How to Start/Stop/Restart Tools in CentOS