Icn airport extremely easy to conx thru and 50 min is ok as long as flights are on time. However if you are coming in from USA or Europe and conx there then it could be a problem as flight times are currently taking 75min-120min longer due to Russian airspace is closed and the other route runs into serious headwinds.
are you checking luggage? you need to check and see if your carrier from Helsinki, assuming Finnair have a lugagge agreement with Vietjet? if not, they would only tag the bag to BKK, and not to SGN...