I planted my vineyard in 1998. I live on a hillside, elevation at 700 ft, with about 3 feet of mostly clay topsoil on top of straight basalt rock. Plowing the existing turf is good, but first you should have someone with a large tractor run a ripper as deep as they can, you will not get a better time to do it, and it is vitally important to help the roots sink deep. That is where they will find their water and trace minerals in the future. I water new plants the first two years, after that they must get water from their own roots, so they sink roots deep to find water in the cracks and crevices of the basalt rock. If they hit a hard pan, they don't have the vitality to do well in a dry summer.