Grass is their natural diet, it contains a lot of what they need. As we generally can’t provide enough grass, then that is why they have unlimited hay.
If you are picking handfuls of grass for them a few times a day for example, then I probably wouldn’t reduce veg.
If they are in a run and freely grazing, then may you want to adjust things.
For example, in the summer mine are physically on the lawn 12/14 hours a day so I do tend to give a little less veg. They still have unlimited hay regardless