Hi Bruno,
VMware Tools is like a set of drivers for a VM. Without them, the VM cannot communicate optimally with the virtual hardware. I highly recommend their installation, but, that's not really going to prevent your VMs from talking to a time server.
This document has some good settings - have you looked at this at all?
VMware KB: Timekeeping best practices for Linux guests
Not being as familiar with *nix as I am with Windows, I wonder if perhaps the system is somehow reverting to the local hardware clock? Maybe you could run a cron job to capture the time source every n minutes and output it to a file? Maybe also run netcat and dump the output to a file? I know I've seen w32tm decide it's going to revert to a hardware or free-running clock from time to time.
Something that may be of interest to read as well:
http://www.vmware.com/files/pdf/techpaper/Timekeeping-In-VirtualMachines.pdf
-Brian