So if I understand you correctly your running out of space on your virtual disk in the VM -is this correct? If it is the space is being chewed up by the operating system and not bye the vSphere environment -
Thick provisioning means when the virtual disk is created - the size of the virtual disk file (VMDK) will equal the size set of the virtual machine when it is created - so the VMDK for this VM should be 20 GB and not change- to get more space you will need to delete files from the VM or you can increase the size of the virtual disk and increase the partition size fomr the VM -
Most vSphere packages do include vMotion -