Not sure what that means.

If you can see the partition in Windows Explorer, and if there is nothing you want on it, just use it as storage. There's nothing to be gained by attempting to 'add it back' to the C drive, and plenty to be lost.

If Explorer doesn't see the partition, then look in Disk Management. If it is accessible there you can format it and assign a drive letter.

If the partition is hidden and protected, then Disk management will fail. In that case, look at the guidance in this link;

Make sure you back up all your personal files first, and also have a plan for reinstalling Vista if anything goes bad.