Create A VM With Only The .vdi file:

1. copy .vdi fil into the path of the vm. you can find the path in VirtualBox Application > File > Preferences > General > Default Folder

2. Create a New Machine
Name: whatever
Machine Folder:
Type: linux
Version: ubuntu 64bit

3. memory: 11264

4. (0) Use an exisiting hard disk file:
    Click on the folder Icon
    Select the .vdi file from the list

5. Click Create

Done - Hope that helps. Below is a quick video