When you sample the soil take several cores from the area in question, perhapse 6-12 cores down to the 6-12 inch level and perhapse to the 2 foot level. Mix them all together and take the sample for analysis from that mix. It will give you an average pH for the area. I would also test for N-P-K, %organics, mainly you are looking for a routine soil analysis. If you tell the soil lab that the crop is grapes, they can usually recommend a fertilizer perscription and an idea of pounds per acre. A prescription rich in sulfur would give the quickest adjustment, however, peat & pine needles would also lower pH over time.