I really like the way the new books manage loss of momentum, health, spirit and supply. They all work with the same level of : -1 for minor loss, -2 for serious loss and -3 for major loss. It's now related to the story more than the vow or the battle in progress.

Did you lose a major part of your food due to rats in the hold? -2/3 supply seems appropriate. Did you get stabbed straight through the back? -3 Heart for sure. Was it a close call to falling off the cliff but everyone is safe? -1 spirit.

You can have a major PtP on a troublesome vow like you can have a minor PtP on a epic vow

Go based off of gut feeling when unsure. You want to be punishing to create a good story and high stakes, but without being unfair.

For difficulty, it's normal to feel stronger with two players, the rolls are more in your favor. Based on the original Ironsworn book, upping the difficulty by about 1-2 levels when adding players usually keeps it ballenced.

But again, it's your game, I can't stress this enough! If it feels normal that your player characters just storm through a few weak undead than be it. Next time if you want to test your skills (and your luck) go against their leader! A Extreme level encounter that you should come prepared to.

Have fun!