Grease it, so it is not so tasty. They are seeing it as an easy source of water so give them an easier source of water than eating your piping. You seem to have some pet rats and don't mind having them around.

Interesting idea. I could try that, but I'm skeptical about the rationale for several reasons.

First, the damage seems to happen only between watering seasons, i.e. in the winter, when it's cold and rainy here. If the rats want water, why don't they attack in the summer, when everything is parched?

Second, I don't think they know enough about drip irrigation to realize that this black tube up in the vines contains water. They probably think it's some kind of branch and they see it as food.

Third, they never seem to chew the small tubes -- only the big ones. The small ones are more numerous and easier to get little jaws around, so they seem (to me, thinking like a rat) like a more appealing target.

I suspect the rats just like to chew things, and for some reason big drip irrigation tubes tickle their fancy. They probably also think the tubes are edible branches, which would explain why they only cause problems in the winter.