I believe it is simbly the Life force absortion that doesn harm the target not the critical hit itself. So you would still do damage and eventually kill the target. And as mentioned earlier they would still need to make a DC 10 fortitude save or die, even if they didnt take any damage.
I wonder if wyroot works on undead though (since they arent immune to critical hits)? If it did you could bypass the fortitude save or die part by making a Rabbit Zombie... And i suppose even if it took damage if you used something like a small light shield (1d2 damage) a critical hit (max 4 damage) wouldn't blast through its DR 5/slashing... Though i guess this would require a strength modifier of 0 (or less)...
EDIT: And it wouldn't need to breathe so you could keep it indeffinetely in your bag of holding when you didn't need it :)
