Got it.
I went to ITH Delhi office with my vouchers but they did not agree to deposit them. So I'll probably have to visit again when I actually have to book the tickets.
I came to know that United very rarely show AirIndia flights.
And I read that there's a phantom availability issue with Vistara in UA, where it will show the seats available but the booking doesn't go through.
Is anyone having a better insight on UA?