Meat Shield strategy huh?
I mean, logically thinking here, that's a vampire. She's likely absorbing power as things go on, if she suddenly kills the third strongest person available in the area, wouldn't that mean she's regenerating that much health and energy?
Plus because it's so sudden, you're really just going to cause the MC to momentarily freeze. He's going to be given a moment of respite because the plot armor and the plot wills it, but logically the vampire would not give a damn and would just follow-up with another attack during that moment of hesitation from the MC as she's gotten a full absorption of fresh blood.
If anything this is the absolute stupidest idea. At the very least an attack to attempt to distract the vampire would make more sense, because the vampire can't take her eyes away from the MC since the MC is the biggest threat, doing so gives an opening.
But being a meat shield like this is counter productive. Also, nevermind having a fist sized hole in your sternum, it's also going through her spine. She should by all account be pretty much instantly paralyzed and quickly dying right now.
If she gets to have a motivational speech to up the MC's morale next chapter uninterrupted by the vampire, I am going to laugh my ass off.