In most cases, it might have been a glitch in the reporting. If VMware says that there is free space, then you have free space. Also, since you are running VMware 6.0 you need to be running firmware version 9.16 or 10.0.1. With running older firmware, you might have had this issue showing in the background & didn’t notice it.
You don’t want to thick provision your volumes. What you can also do is to run the reclaim space command on every volume and that will reclaim any unused space from your volume. When you run the reclaim space command it can take some time to complete. Here is a link to the VMware KB that states how to reclaim space. https://kb.vmware.com/s/article/2014849
Please let us know if you have any other questions.