It's wonderful you rediscovered your father-in-law's vineyard! Neglected vineyards can often be revitalized with some time and care. Here are a few tips:

Prune out old, dead wood to open up the canopy and improve airflow and light penetration. You may need to do this over several seasons. Go slowly to avoid overpruning.

Train vines back onto the trellis system. Cut back upright shoots and use training wires to guide vines back into position. It may take a year or two.

Control weeds/grass in rows. Mulch or cultivation helps vines compete.

Fertilize to stimulate growth after pruning (early spring). A balanced 10-10-10 or organic fertilizer works.

Monitor for pests/diseases and treat if needed. Good airflow after pruning helps reduce issues.

Be patient! It takes time to bring old vines back into shape. Don't try to do too much at once.

The most overgrown vines may need to be fully renewed by cutting back to the graft union to retrain new growth. But less overgrown vines can likely be salvaged with gradual remedial pruning. The grapes this year are a good sign the vines are still productive. With proper care over time, the vineyard has good potential for restoration. Let me know if you have any other questions!

Think you so much for the response! We’ll keep our fingers crossed we can revive it! 
wendy