Maybe it's just my personal taste, but I never liked the idea of Vorpal killing a troll or a hydra. Some things can get by without a head, and I don't think vorpal should bypass that.
No, reading everyone's comments, I'm wondering if it's best just to let it alone. Too many things are immune to crits, and it makes it such a situational advantage (similar to Brilliant Energy). The best suggestion I've seen so far was to force a Coup De Grace on a crit, but I'm still not sure if I want to implement that.