Do only Dark Knights acquire vices? 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.