Just for information, I am on solid bedrock and have my test plot on 2 feet of topsoil I imported and placed with my backhoe. I had a good yield on 25 vines this year and have moved on to the main vineyard with 100 vines under the conditions you speak of. (Yours are actually better). Think of the vines in this case as giant potted plants. While they would prefer deep soils, if you do a soil analysis and follow the guidelines provided, you will do fine. The big issue is driving posts for a trellis. I wound up drilling them in with a compressor and rock drill, no fun, but they are solid. Hope this helps.