'vmkfstools' can be used for different tasks. Cloning a virtual disk from one datastore to another, without changing the disk format is actually the same a copying it. However, 'vmkfstools' works differently compared to e.g. the native 'cp' command.
'vmkfstools' allocates the required disk space first, and then starts copying data blocks. The 'cp' command (as well as other tools) allocate disk space on demand (i.e. the file size on the target grows with copying data), which puts a high load on the VMFS file system.
André