Although RVTools is a very reliable tools, I would suggest you check the VM's log files (vmware.log) to see whether the .vmdk files are referenced. In case you have active snapshots (i.e. "<vmname>-00000X.vmdk" file names in the VM's HDD settings you should also be very careful. In any case, rather than deleting the files right away, I'd consider moving them to a new sub-directory in a first step, and - if everything continues to work as expected - delete these moved files at a later time.
If you're still unsure if they are really "zombie" try first move them to a different folder and let them there for some days/weeks until you're 100% sure that nothing is using these vmdks.---
Senior Infrastructure Specialist