I applied for the Turkish e-Visa. Since I have a Vietnamese passport, I am required to have round trip flight ticket with few specific airlines (Turkish Airlines or Pegasus). Just wondering what round trip means in this case? I want to fly into Antalya and fly out of Kayseri, both with Pegasus. Is this considered roundtrip? Or I need to fly in and out of the exact same location (e.g. Atanlya).
The requirement for a "round trip" ticket is really just short-hand for saying that you need to hold a ticket to leave the country. There is no need for this ticket to leave from the airport as you arrived into, nor even that it takes you back to the same place that you arrived from.
The only additional requirement is that as well as the ticket to leave the country, you will generally also need to hold the necessary documents to complete that trip. If you are returning to your home country, then obviously that's not a problem. But if, for example, your onward ticket out of Turkey was for a flight from Kayseri to Madrid, then you would also need to hold the necessarily visas/etc to enter Madrid (which might in turn include an onward ticket from Madrid to elsewhere!)