So the girl getting in the way being a meat shield basically cut off his strategy to prevent the vampire from escaping and all he got as a trade off wasn't a fatal blow, but just the removal of one arm. Which as this is a vampire, you would think they would have regenerative capabilities. And when magic is involved that can do something like cure a giant hole in your chest and even have the ability to reverse scarring tissue if you go to an actual healer mage. Yeah, logically what happened was a wash.
Again, due to a brain dead tactic from the meat shield princess (Technically meat shield duchess/noble, but meat shield princess was the first thing that came to mind as I wrote) who thought her giant booba would somehow be able to cushion an attack, even though she didn't properly face the enemy and instead offered her back to it, which if she is a warrior. Scars on you back are a sign of a coward. She would and should have been facing the opponent instead of facing the MC. And since she was either coming up to the MC from behind him or from the side, it took invested effort for her to look at the MC, which I can understand she's overly infatuated with the guy so she probably wanted to look at him while she took the hit. But that opens the way for mental and emotional scarring. The same would be true if it was her back, but now the MC would have her shocked face tied to her taking the blow.
She's just lucky the guy is too dense to gain any new PTSD. And again, there are better, more intelligent, more strategically viable ways to create an opening. And his assessment was that he was one step short of killing the vampire. Likely he probably would have if she had done a proper diversionary tactic, rather than actively get in the way of combat. Her dying body also doubled as a sudden dead weight halving the movement capabilities of the MC and created additional issues.
I'm both glad and sad that she didn't do some stupid talk no jutsu uplifting speech as she was stabbed by an arm and got her free action of talking mid combat to motivate the MC. But it also does not eliminate the fact that what she did was nearly the worst option. Worse than that would have been blocking the attack with her head instead of her chest. Or to attack the MC with a tackle to get him out of the way and completely unbalance him mid combat.
Well, combat analysis and frustration with that aside.
Congrats ya dummy. You're now on the fast track to be the grand emperor of the continent if not the world based on the measuring stick of you got a whole damn arm while someone else just got a finger.
Hell, some people struggle to even just get the nail off of a vampire.
... So what're the odds that the vampire eventually becomes a part of his harem? After he defeats her of course.