I think its kind of worse than tanking it. The way its explained, if she didn't dodge it she would have literally fucking died from that attack. They foreshadowed this sort of thing a while ago where that one dude told she she was basically putting her life on the line in every fight because of her retarded fighting style. Isn't it great that Vivio almost made her own mother murder her in front of thousands of people?
I have to a agree, the explanation is kind of nonsense. I don't even know anymore if the damage is still supposed to be simulated in these battles since in the past we have gotten contradictory information from the story, so it isn't certain if she was really at risk.
Finally over, a last Vivio lose because win against Nanoha will be unacceptable, After 100 chapter,they never confirmed Nanoha x Fate relationship and even this chapter reforced that Nanoha is the real mom while Fate is more like a extra.
I don't think I should take anything posted by Mugino in this thread at face value, but in canon Nanoha and Fate are married at this point and Vivio is officially and legally their daughter.