Huh? What do you mean separate PNR for Vistara? That flight should appear on the same reservation as your DEN-DEL flights, but should show have a separate confirmation number for Vistara. Is that what you mean?

Regardless, this is just standard United.com nonsense. If you can get the itinerary to come back from Google Flights, they often provide a booking link to United that would have worked. Otherwise, when the website is acting like this, calling to book is the correct procedure. Waiving the $25 is standard if you explain that the flight couldn’t be booked online.