Nitrogen deficiency is the other probable nutrient problem for leaf yellowing in grape. The symptoms looks similar to iron deficiency. Depending on where you are, most US state university extension services offer plant nutrient testing. Call or google your state's ag extension office for instructions on how to submit samples. In general: For grape, you'd cut several leaves off, including the petioles, refrigerate them, and send them in - but each state has different specific instructions. They test the leaves and tell you if iron or nitrogen levels are low and if/how to amend your soil.