Yeah, it's an issue with the third-party editor in vBulletin. We know there are a ton of issues, and we recommend sticking with the standard or basic editor (not the WYSIWYG one), but unfortunately there's not much we can do for now.
Something we've noticed is that if you see the cursor disappear, drag the box at the bottom right hand corner bigger, and you should get your cursor back and everything should be fine (for a while).
We're waiting on a real fix from vBulletin/CKEdit, so hopefully that will come soon.