grass is more appropriate than any vegs, don't worry. It is their natural food, good for their gut and for developing the good bacteria. My piggies eat hay and grass, then some piece of bell pepper, but the grass is their basis. And they are growing healthy and strong with a good quality poo and wee.
Don't worry...
of course the grass is not available all year long and they have their strict preferences and tastes. But now go on like that. The vet will be satisfied.