It seems that I only ever hear Ranta talking about getting the vice from a monster's kill. I understand that by getting these he is able to level up special skills related to his demon. Are Dark Knights the only class able to acquire vices?
Short answer: yes.
Slightly longer answer.
In the beginning of the Anime, when the group including Ranta go to get their class specific training, We are greeted with Ranta having a new ability with him. The 'Vice' you mention. This is because, as we know 'Dark' knights, offer up sacrifices to some form of a 'Dark lord' and have formed a pact. Just like the Protag (I forget his name) got the ability to see an enemies critical location, and follow a 'stream' of light with his blade, to reach it. Ranta got his own class specific ability.
To answer your question, yes. Ranta is the only one who talks about vices as he is the only dark knight in the party. Vices are exclusive to Dark Knights. I would think of them as class-exclusive perks found in numerous (MMO)RPGs.
I would provide more information but the only relevant information conveyed in the anime is that:
When a dark night kills a foe, they can take a part of the enemy with them, be it a toenail or a heart, then they can use these parts to get new perks, like Demon Companion.