I saw the response for converting VDI images into VHD images, but I want to do the opposite conversion - is there a tool to do this?
Adapting one of those answers, you could use VBoxManage:
VBoxManage clonehd source.vhd target.vdi --format vdi
If you are just looking to use the vhd in Virtualbox, you do not need to convert it yourself, Virtualbox will quite happily use a vhd image. You can just add it to the list of disks in "Virtual Media Manager".