It's supposed to be a dark, gothic type of game.

What types of mechanics does it use?

I don't have Vaesen, although have been eyeing it for awhile because Fria Ligua makes terrific games - Symbaroum and Coriolis are great, at least conceptually and in terms of production values. Not sure about the game system, but the books are chock-full of great ideas and the art is second to none. Plus they're taking over the One Ring, which is a good thing.

I have Vaesen, got my PDF right here.

It is a dice pool game. Add Attribute + skill and roll that many dice, count 6 as successes.
You can have Talents that add dice in certain circumstances.

Initiative is handled by drawing cards from a deck of playing cards, using the ten cards from 1-10 in one suit.
Lowest goes first, but you can swap cards with another player if you feel its better that they go before you.

Once you get hurt in combat, you only have 4 physical condition states, Exhausted, Battered, Wounded, Broken.
You also have 4 mental condition states too. Angry, Frightened, Hopeless, Broken

Combat is rarely the most efficient way to fight Vaesen though. By performing certain rituals from folklore or religion most Vaesen can be driven off without a need for fighting.

Not a huge fan of Free League's dice system, however, I am a major fan of their settings, both Vaesen and Forbidden Lands.

Basically, you have some sort of Scandinavian horror at work. Usually research or careful interactions with NPCs with guide you to the creature's weakness. Each of the creatures makes for a great one shot. I ran the Church Grim (using a different system) and it was fantastic.

A side benefit of playing with the settling is it helped me think a little differently about how to leverage monsters in games, even human villains.

We ditch the Zero system, too. We just told play snd look at stat numbers, if that makes sense. Sort of diceless.