In case of the 2GB split disk format, your virtual disk consists of multiple data files (e.g. <vmname>-sNNN.vmdk) and one header/descriptor file (<vmname>.vmdk) which describes the virtual disk. In the vmware-vdiskmanager command, it's only the header/descriptor vmdk which you need to supply as the source virtual disk.

Convert to a pre-allocated disk.
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vmware-vdiskmanager -r sourceDisk.vmdk -t 2 destinationDisk.vmdk

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The following line is simply changing a pre-allocated input disk into a growable target disk.
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vmware-vdiskmanager -r sourceDisk.vmdk -t 0 destinationDisk.vmdk

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