A draft system has now been implemented for private messages and posts. Posts and Pm’s over 300 characters can be saved. To access the draft after you saved it just go back to the thread and hit reply and it should already be here. For pm’s just press new message and your draft should be there as well.
You can save one reply per thread and one draft pm. The drafts will be automatically deleted 7 days after creation.
This feature does not currently work for threads. However the updated version that only works with vBulletin 3.8 should allow for saved threads so it will be implemented once the forum is updated.
In case you were wondering where the button is attached screenshot.