What's REALLY nasty is if a poisoned weapon causes a critical hit, and target fails his Fort save. This will probably kill the target, whatever it is.
For instance, see Smaug, a honking big red dragon with a Con of, say 23. See little human named Bard (despite his name, he's really a Fighter, but let that pass for now!) praying that his Black Arrow (with Black Lotus Extract) hits. See Black Arrow score critical hit for, say, 8 points of wound damage. And 3d6 Con, in this case 15. Good roll, Bard! Smaug falls down, smashing the local real estate. Smaug is disabled. Since the local real estate was built on a lake, Smaug drowns.
(In fact, I could argue that Smaug drowns even if that poison roll wasn't so good. Reducing him to half WP would probably keep him from flying, and once he's in the lake he's pretty much done for!)
Yes, Bard had lots of luck (critical hit, dragon blew Fort save, good poison damage roll, convenient lake), but he still brought down quite a bit a dragon with his one arrow!
Which suggests another possible setting for the VP/WP system...