Reports of problems with Vanish often plague the forums. Most often these issues are caused by client/server latency, as Hortus reminds us, but this time they've identified a separate problem. While not disclosing the nature of the bug, Hortus has informed the rogues in the thread that their feedback has resulted in the discovery of a problem with these two skills and they're currently investigating the issue in hopes of finding a fix.
It's nice to see that players are able to help contribute to fixing the game. If you've had any problems with these skills recently, I'd suggest that you read over the thread and see if your experience adds anything to the discussion.