How to convert VDI to VMDK; Converting a VirtualBox image to a VMware disk image and vice versa

You need the QEMU emulator software package
aptitude install qemu

in order to execute this command
qemu-img convert VirtualBox-HardDisk-image.vdi -O vmdk VMware-virtual-machine-image.vmdk

You should keep in mind that you might get an IDE VMware disk image in this way but a SCSI hard disk image is recommended by VMware for more I/O performance.

Just type

to get all supported formats which you can supply as the -O parameter. It is also possible to convert a VMDK to a VDI hard disk image. To convert a VMware image to a VirtulBox image, you have to use the qemu-img-vdi command which you can get by patching QEMU.
Afterwards you may execute this command:
qemu-img-vdi convert VMware-hard-disk-image.vmdk -O VDI VirtualBox-image.vdi