Don’t bother – VirtualBox now natively supports .vhd drives – how handy is that? =D
But if you really wanted to, just use:
VBoxManage clonehd source.vhd target.vdi --format vdi
This will place the .vdi version of your drive in your current users VirtualBox folder, which is: ~/.VirtualBox
Relatedly, to go the other way (.vdi to .vhd) you could use:
VBoxManage clonehd source.vdi target.vhd --format vhd
Note: For this to work, you must not have the drive attached to any virtual machine, so if you already have it connected to a virtual machine, detach it first, mkay?