Both vdi and qcow images are compressed, so a a 10 Gb .vid or .qcow will take less space on your hard drive. Conversion, however, requires you to first convert the .vdi to a raw image, which will require the full 10 Gb, so plan accordingly.
Assume your VirtualBox image is called “ubuntu.vdi”
Step 1. Convert the VB disk vdi image to a raw image format.
#VBoxManage clonehd --format RAW ubuntu.vdi ubuntu.img
Note: The ubunut.img is a raw disk image, it will be quite large (as many GB as your virtual drive is, see above).
Step 2 : convert the raw image to a qcow
#qemu-img convert -f raw ubuntu.img -O qcow2 ubuntu.qcow
Upload the ubuntu.qcow to web/ftp server and Import it as appliance from VxDataCenter