You definitely want to mulch your grapevines. They'll bake if the soil is exposed to our unforgiving summer sun. However, peat moss and its derivatives break down quickly in this climate. Don't expect that peat mulch to last through the growing season. The same holds true for the peat in the soil mixture; it will last only a few months. If it's still possible to work amendments into the soil, coir (compressed coconut fiber) is excellent for moisture retention, and in Texas it has greater staying power than peat. I'd add a couple of inches of good compost to the surface to replenish the nutrients the soil microbes devour as our summer temperatures rise. Frequently replenishing this top dressing will keep the bricks at bay. Cover the top dressing with a dense layer of shredded bark mulch, e.g. cedar or hardwood, and your grapevines will be off to a good start.

I'm no expert, but that seems like too much greensand, which may explain your results. A 40-lb bag is used to topdress 1000 square feet...

Yes, that's too much greensand. Greensand is used as an amendment not as a soil. Sand + clay = brick.