For this purpose I used following commands on my Ubuntu 11.04 machine :

First you must install qemu :

mickey@mickey:~$ sudo apt-get install qemu

Then convert .vdmk to binary

mickey@mickey:~$ qemu-img convert image.vmdk image.bin

And finally convert it to to .vdi image :

mickey@mickey:~$ VBoxManage convertdd image.bin image.vdi

Happy coding!