Modern Kevlar is notoriously poor against stabbing weapons like knives, or swords that matter. Problem is getting in range of said rifleman. There IS a reason once semi modern firearms (a black powder rifled flintlock) things like pikes were abandoned with unseemly haste. Especially with the advent of the ring bayonet. Swords stuck around with cavalry for a bit longer, but, they went the same way in the end. Look into the Battle of Najaf sometime, it involved some pretty intense hand to hand fighting and the Marines bayonets were used in full force. Especially in between the graves, the marines couldn't see what was around every corner and had to clear it out the hard way.
http://en.wikipedia.org/wiki/Battle_of_Najaf_(2004)
Granted this IS a manga so Katana tends to trump fighter jet. But the author in this has tried to keep modern weapons capabilities in a pretty approximate ballpark.