To convert a Virtual Box vdi file to a VMDK file you can run the command below.
[root@localhost]$ vboxmanage clonehd CentOS_5-5.vdi CentOS_5-5.vmdk –format VMDK
0%…10%…20%…30%…40%…50%…60%…70%…80%…90%…100%
Clone hard disk created in format ‘VMDK’. UUID: 7e2d98da-2cb7-4442-8027-10cec043c83a