I have no idea how they get their data but every single flight I have taken in the last few years has been exactly the same as flightradar data.
i think you are completely overthinking the whole vietjet thing, yes they are far from perfect but there is the data there for that particular flight to show that there has been zero cancellations for the last 3 months. If you're worried about booking it with them then maybe look at a different carrier and pay the extra they are likely going to cost.