United Airlines has established a codeshare agreement with New Delhi-based Vistara, in which United will put its code on more than 20 Vistara flights. The agreement, which takes effect in the fall, includes reciprocal frequent-flier benefits for United MileagePlus and Club Vistara members. United has daily nonstop flights between Newark and both Mumbai and New Delhi, and it will add seasonal service between San Francisco and New Delhi on Dec. 5.