You're not going to be able to delete unless you A.)mount up the vmdk to another machine or B.)boot into WinPE or recovery mode. Documentation says to set it to the size larger then the VM memory. I set it to the right size and left it on the parent.
Ok thanks, it doesn't look an easy process. Our parent machine is modified every 2 weeks, so I prefer losing 16 GB on datastore rather than deleting the file manually before everey recompose.