Is killing vermin and insects justifiable? The first precept goes something similar to 


  I undertake the training rule to abstain from killing.


Now, I understand that, for instance, eating meat is allowed, but what about, for instance, killing vermin which might damage food or cause disease (such as mosquitos in places where malaria is common)?

Is killing justifiable then?
The beauty of the precepts is that they are impossible for ordinary human beings to fully observe.  The precept against killing is an example of this. My body's immune system kills all lifeforms that it doesn't recognize.  I step on and crush innumerable microorganisms.  But the first precept asks us to minimize the amount of harm we do in the world.  In the teachings of Thich Nhat Hanh, the precepts are trainings, not commandments.