Use vmware-vdiskmanager to create a copy of the disk file. This works with flat or split disks. vmware-vdiskmanager is part of vmware server so you will need to install that first.
vmware-vdiskmanager -r Ubuntu.vmdk -t 0 Ubuntu-copy.vmdk
Note: If you have multiple disk files the first argument is the BASE NAME of the series.
Once you have the copied image complete that file can be used to convert into a raw disk format usable by KVM, Xen, VirtualBox, etc. etc.
qemu-img convert Ubuntu-copy.vmdk -O raw Ubuntu-copy.raw