While I feel that the design was a little bland, you have to understand that it is a tool for killing. Vergil isn't going to a ball where he has to elegantly slice demons and get their feedback from them. A true warrior can pick up many types of weapons and be skilled with them. A design of something doesn't make it badass or cool, the wielder does.
We see weapons like this, and mostly our common sense say that "it's just a tool for killing" just like some things or aspects seems meaningless to some people who do not deal with them in a daily basis, like a programming code means nothing more than "computer stuff" to someone who doesn't understand computer science. This is why the warrior's mindset towards weapons is a little different, just like our clothes in modern days can tell a bit of who we are, for warriors dealing with other warriors their weapons could tell "who" they are, i'm getting a bit too much philosophical but lets try to show this in a more consistent way considering the mindsets of both Vergil and Dante:
Why could Vergil disdain firearms as a weapon of a "true warrior"?
Because he thinks that anyone, even with little MOTIVATION, skill and dedication can point a gun and pull a trigger roughly efficiently? Could be this what Vergil values in a warrior: dedication and skill? If it's true it becomes kinda clear that Vergil does not sees weapons just as "tools for killing", and sees a meaning in combat itself, seeing that a gun even in their universe can be just as useful as a big sword to slain demons and other things: this opposing between swords and guns being "true warrior's" weapon appeared even in human history in Japan during the Meiji Restoration for example, some samurais just refused to take a gun instead of a sword and started to be wandering swordsman looking for others with the same way of life to battle, test their skills and eventually die in a way they understanded as honored and brave; some other samurai decided to take on the "art of the riffle" (Teppo Jutsu) as another way of living as a warrior, and understanded this to be so honored and brave as the older one.
Dante doesn't have this same mindset and mostly "prove" that it's possible to make a skillfull use of a gun in battle, just as refined as you can get with a sword (in their own universe, obviously). I think their "opposing" in this terms of "what makes a warrior" comes from the "traditional vs modern" but they have similar values and morals behind this opposing: both the "traditional" and "modern" warriors in the end of the day needed to dedicate themselves to their very own arts and can take the path of a warrior from both ways, even with their disagreements. They have a common ground that make both of them warriors, that is the joy and meaning of battle by itself.
Vorgil and Donte does not take this "warrior mindset" forward, in the end of the game it becomes clear that fighting and combat in general are just as you said for them: ways to eliminate opposition, instead of a path of self-improvement, joy or a way of living that they want to keep or that they think that is impossible to extinguish. They want to bring peace (and in Donte's case, freedom) engaging in combat those who opposes their beliefs.
TL;DR
Vorgil and Donte sees combat as a way of reaching goals primarily, they not take combat as their duty and way of life and are portrayed more as these "political leaders" in a bigger plot, with their own beliefs in what is better for humanity. Dante and Vergil seeks combat and sees meaning and joy in combat itself, they take their duty as warriors and are portrayed as such by their creators. This is why i think "DmC Vergil doesn't look like a warrior and classic Yamato was meant to represent a warrior's weapon, so they don't fit too much."
