Volkswagen has agreed to pay at least $1.26 billion to fix or buy back nearly 80,000 polluting 3.0 liter diesel-engined vehicles — and could be forced to pay up to $4.04 billion if regulators don’t approve fixes for all vehicles, court documents filed late Tuesday showed. In December, VW said it had agreed to buy back 20,000 vehicles and expected to fix another 60,000.