Use the following Virtualbox command in your shell:
VBoxManage clonehd old-file.vmdk converted-file.vdi --format VDI
tested with: Virtualbox Version 5.0.24_Debian
Use the following Virtualbox command in your shell:
VBoxManage clonehd old-file.vmdk converted-file.vdi --format VDI
tested with: Virtualbox Version 5.0.24_Debian