The three virtues and the three vices have a profound effect on your combat abilities, tools and weapons, as has been pointed out before. Basically they are your stats that determine your effectiveness against a character and the strength of the book or weapons you use. You enemies also come with their own virtues and vices which should help you determine how to fight them and with what.

For example, if you use a sword, it’s Major Vice could be Destruction, with a certain level of Nihilism and Disincentive theses stats add to the level of the vices you have already accumulated in the game and have and is balanced out against the enemy you encounter depending on their stats.

In turn, books are used for writing and they too have a certain set of stats in Virtue and work in kind.

Writing doesn’t kill, it inspires the enemy to respect you.

How ever fighting is destructive in it’s very nature and is more powerful than virtue in the game and will make your encounters easier to deal with, but it comes at a price.