No - there is a new rule: You can "transit for 72 hours visa free" since you are going COUNTRY 1 (Japan) - KOREA - COUNTRY 3 (Vietnam)
Because you are flying VietJet Air - you MUST enter Korea through immigration and customs - because you have to go out and visit the VietJet check in counter to get your boarding pass. Tway and VietJet are both low cost airlines that don't offer any sort of transfer assistance/transfer desk service.
https://www.hikorea.go.kr/pt/InfoDet...showMenuId=375
Check the "No Visa Entry for Foreign Transfer Passengers."