Can I buy tickets on the Vietjetair website with a Russian card?

No, this is not possible with either a Russian card or a foreign card, as there are simply no Unionpay cards among the payment methods. When trying to enter a Unionpay card number, you will simply receive an error "Card Number is invalid". And no VPN or other tricks will help here, no matter what is written on various dubious websites. Therefore, as an alternative to buying tickets on the official website, use other websites that accept Russian cards. A white (verified) list of such websites is here - https://nashaplaneta.net/besopasnost/kak-ne-popast-na-moshennikov

How much does a visa to Vietnam cost?
Where to go in Vietnam for the first time?
Where is best to buy tours to Vietnam?
Follow the link, all these questions are discussed in the chat - https://t.me/vietnam_travel_chat