I've tried numerous time to pay on Vietjet's website. It never worked. I contacted my bank and they told me that the payment went through so I had a lot of floating authorizations on my credit card.

I only was able to book through the android mobile app. For some reason the payment does go through there (same credit card). But on the mobile app, it's not possible to add priority check in (which is worth it in my opinion).

I hadn't tried to email them but tried calling their customer service. They were less than helpful.