It depends on how much soil you will pile on top of the grass. If you add 12 inches or more of new soil, just cover the weeds and grass with a wet newspaper barrier and pile on the garden loam. The grass will die and the roots will grow right through the paper. If you are adding less loam, I would dig and remove the weeds and grass so that they won't pop up and fight the vegetables for space in the garden. Great soil is at the root of all great gardens so be sure to fertilize your vegetable garden even if you invest in garden loam and compost. I suggest an all purpose slow release plant food like Osmocote because vegetables need both micronutrients like copper and magnesium as well as the big three such as nitrogen, phosphorus, and potassium. Keep growing, Marianne Binetti