All in-flight services are subject to a fee, but the prices are relatively fair. A hot meal (a small box of rice or spaghetti and meat) comes at around 2,50 usd, or from 4,50 usd at the Thai branch of VietJetAir, you just need to order the service in advance online during the ticket buying process.
Choosing a seat on the plane will cost roughly 1 to 2 usd, if you refuse to pay, you will be randomly assigned any seat for free.