Interesting, the author clearly is a gun enthusiast but being Japanese likely only has second hand information. I think you or him made a typo, the sniper likely said Sig p320 instead of 230 as the latter is not a 9mm pistol. A few thoughts:
1. 30m is a very long distance for a pistol, far longer than is drawn in the manga (relative to their height, they are drawn only about 10m apart). At that distance an untrained person has basically zero chance of landing a shot. However it would be fairly easy with a rifle. The author likely knew this and wrote the conversation around it.
However, for a trained gunman, hitting a human sized target at 30m should be fairly easy if you aren’t under pressure. In this scenario where you’re against body armor, it’d require very good marksmanship to land a head shot at 30m, but completely doable. Against a moving target though I doubt anyone would be able to make it. So if the guy had just shot before announcing his presence, he’d probably kill assuming he’s a good shot. After it becomes very hard assuming the opponent will try and dodge. I’d say realistically vs a still target I’d have 100% hitting the body, 70% hitting the head, and 1% of getting a headshot against a moving target
2. Level IV body armor is the highest tier of commonly available body armor, capable of stopping multiple high caliber rifle rounds as the author mentioned. However it is exclusively made out of metal or ceramic plate, and there is no soft armor version that is even close to being developed afaik. It is also complete overkill outside of a war zone, level II, which is what soft armor you’ll generally see police wear is rated at, is sufficient to stop all non armor-piercing pistol rounds (which are rare, most people will use hollow points).
I also don’t think such material is flexible enough to form a hooded jacket. Most body armor only covers the chest to protect the vital organs there. The stomach, hips, arms, etc. are generally not covered because you’d have a hard time moving. Also a crotch shot would be very effective here as it is not armored unless you don’t want to move your legs very much, and it is close to your center of gravity so it’s fairly easy to hit.