I came back from Nepal few days ago via Qatar Airways. Proof of vaccination is not a requirement. Proof of negative PCR test result conducted within 72hrs of the flight is required. PRC test result is checked at 4 times in TIA and 3 times in Doha. An announcement was made while i was waiting for immigration here in US that CDC folks may randomly ask to see PCR test results. Make sure PCR test is done in GOVT approved test centers. I did mine in Mediciti. You can find approved test centers in CCMC website. Kathmandu private hospitals give same day results so i did mine a day before my flight. CCMC (https://ccmc.gov.np/arms/person_add_en.php) needs to be filled and barcode number be handy. Good Luck