Vistara had signed the codeshare agreement with SIA and SilkAir in 2017 and expanded it this year in August when it started International operations.
Bookings under Vistara's Grand International Sale are only open from October 21-23 for international round trips to Singapore, Dubai and Bangkok.
With the addition of daily direct flights to and from New Delhi, Thiruvananthapuram became Vistara's second destination in Kerala, after Kochi.
As an introductory offer, Vistara is offering all-inclusive, one-way Economy fares starting from Rs 3099 for Delhi-Patna and Rs 3686 for Varanasi-Khajuraho.
Air Vistara, however, quickly responded to the matter and apologized for the discrimination.