hanns.g: As far as I know (and I will eventually test it), Riposte only works with humans and requires the use of block. Nonetheless, it is EXTREMELY powerful. In that respect, it seems like a fair trade in vigor, being such an offensively powerful ability. Even without Riposte, blocking causes staggering and an opportunity to attack. Dodging and rolling have no offensive aspect (well, if you get in position, you can backstab, but pretty rare).
So, it makes some sense to me. But, that argument falters a bit when you consider throwing takes no vigor. :(
Edit: Yeah, I figured Riposte would only apply to sword-wielding opponents. Blocking doesn't work very well against beasts either, since they hit so hard and tend to have combos.
I'm pretty sure Riposte works against monsters too. I remember counter-attacking harpies. Blocking works decently against beasts (well not against a fast-attacking mob, but I have used it on occasion against Endrega Queen)
Dodging and backstabbing works like a lucky charm against large creatures and slow opponents (esp. shielded humans or armored knights).