If I have said above power that gives me a critical hit range boost as an effect, and is reliable, then I have no reason -not- to as the first thing I do after every rest, and every five minutes after that, like clockwork.
I mean... after all. It's an effect, right?
I can see how this isn't something you want happening in your game. The power's authors shouldn't
1 have used an
Effect: line but instead included it as part of the
Hit: clause (and a DM could houserule that). Alternatively, you could rule that on a natural 20 the power is expended by virtue of being an auto-hit - after all, part of the problem with the crit-enhancing power is that it's reliable and you're avoiding paying the cost. That particular twist doesn't apply to most other powers, in particular, the OP's teleportation power is not reliable and using it means expending it.
If you
do permit this, your game balance won't break - critting on 19-20 is pretty standard by 29th level, and Vorpal Doom (the power in question) comes at the opportunity cost of not being able to pick other (mostly better) 29th level daily powers. Not that I think you want this happening...
I also have to point out that by 29th level there are numerous other ways of guaranteeing or almost guaranteeing a miss, so this trick too could be triggered by attacking an ally. Attack penalties, defense boosts, encounter immediate interrupt forced movement or teleportation, halfling second chance, basic things like shield push all mean that the problem certainly doesn't
exclusively lie with the ability to gain the miss effect by attacking an empty square - it lies with the miss effect providing an encounter-long buff and being part of a reliable power.
Since Vorpal Doom's benefit isn't that large, only affects levels 29 & 30, and the exploit is clearly abusive, I don't think you'll run into players actually doing this very quickly, but if you do, I admit I'd probably try to fix it.
1Generally speaking, reliable powers with a miss effect always seemed suspicious to me anyhow... and this is a good example why...
Edited to fix the confusion Flipguarder tripped over...