I would recommend testing the leaves/petioles to see what your vines are lacking. Soil testing is great prior to planting and should be done, but once the vine is growing, let the vine tell you what it needs. As for amending the soil, the testing facility can help. I don't recommend using a general purpose fertilizer as it will not address your needs totally and maybe over dose your vine in some areas. There are products you can probably buy at a local nursery or hardware store that will cover most of your requirements. I mix my own based on the test results from the previous year, then amend around the same time I prune (or prior to bud break). Grape vines are hungry little devils, so it's an ongoing process. If you have a small operation, it doesn't have to be a big endeavor to do this. The testing folks I use run about $12 a test, which is all you need for a small amount of vines. Just remember to take a couple of leaves/petioles from each vine so that you can sample across all of them. Supplements can be bought in small (usually 4 or 5 lbs) quantities and the prices are not terrible.