>It was my assumption that the change was made primarily to
>prevent ass;vanish, but I'm realizing that's probably
>incorrect.

Sort of. ass; vanish might have called to our attention the previous state of the skill; I forget.

Basically, once upon a time vanish worked half the time in combat.

Sometime later, I think around when assassins got their last big revamp, Cador talked to me about that, and he got me to agree that in the light of various changes that had taken place, it should be a little more reliable than that.

Sometime later I noticed that 'a little more reliable' had become 'never fails' and redressed it.

It now works somewhere between half the time and all the time in combat.