You're absolutely required to protect them from each other. It's called "keeping the king's peace" and all that junk.
Now, how that actually plays out is a matter of power politics. But if you can't, in general, keep them from fighting each other, you're a laughingstock of a liege.
In Project Balance, the King's Peace is mentioned, but vassals hate you if you try to implement it, strangely enough. No King's Peace give a minor opinion boost, so I take it that they want a free hand on each other's lands, and I, as a liege, am there to tend to the fence.
Well, as long as my vassals give me the troops I want, I would want to have a word with those who are laughing....