How To Convert VMWare Image (.vmdk) to VirtualBox Image (.vdi) on Linux

First we need to install QEMU

QEMU is free software written by Fabrice Bellard that implements a fast processor emulator, allowing a user to run one operating system within another one. It is similar to projects such as Bochs and VMware Workstation

sudo aptitude install qemu

Now using qemu, first we need to convert a .vmdk (VMware image) to a .bin format, which can then be converted to a Virtualbox native .vdi format.

qemu-img convert /path/to/original.vmdk converted.bin

VirtualBox is a free, powerful and versatile virtualization program which is available for Linux, Mac, and Windows hosts, and can virtualize many different Operating Systems.
VirtualBox was originally developed by Innotek, but was purchased by Sun and renamed Sun xVM VirtualBox.

Now using VBoxManage utility that comes with Virtualbox we can easily convert the .bin file that we have generated using qemu to a native .vdi format:

VBoxManage convertdd converted.bin converted.vdi