If some mage/whatever beat Nanoha, then why does Vita look fine? I don't think the situation could have ended like that if it was due to a fair fight.
For the scene to properly tie into the events at the end of ep 8, I think it would have to be
partly due to a mistake on Nanoha's part. Something like, not paying full attention while on a battlefield.
Vita seems to blame herself for the events so I'm betting on something like: Vita and Nanoha are on the battlefield together, and maybe after it seems they'd finished all the enemies, they both relaxed. Then a shot came out of the blue towards Vita and Nanoha saw it first and jumped in to shield Vita. Vita is unscathed, but Nanoha is badly injured.
Personally, I'm more curious as to what Nanoha decides to do with Teana and Subaru. Realisticly, I don't think a simple apology from either would be enough - since they haven't learnt their lessons in the past, they aren't trustworthy right now. Basically, they need to learn (and control) fear of death - or at least, fear of Nanoha
I'd suggest maybe something like: Playing a "survivor" mock-battle with Signum - ie Teana and Subaru's goal is simply to survive long enough against Signum going at them directly (though probably have to tune down the physical damage). After being "killed" repeatedly by Signum, I think they'll learn...