Convert a VMWare Image to VirtualBox Image

Convert VMWare image called centos.vmdk to /tmp/centos.bin

$ qemu-img convert centos.vmdk /tmp/centos.bin

Now use VBoxManage to get back image in native format:

$ VBoxManage convertdd /tmp/centos.bin centos.vdi

via How To Convert From a VMware Image To Virtualbox Image.