For converting Vmware Image to Virtualbox Image you need to use the qemu-img tool. It is quiet easy to use. After installation of this tool you can simply convert a VMWare Image to VirtualBox Image. For converting the VMWare image to Virtualbox consider following example.
Convert the <filename>.vmdk to <filename>.bin
Now use VBoxManage to get back image in native format which is virtualbox
qemu-img convert <filename>.vmdk <filename>.bin
VBoxManage convertdd <filename>.bin <filename>.vdi