Even though Korean Air has many flights to Hanoi from different origins in the world, they are only indirect flights that have a stopover in Seoul (Korea) before the passengers can reach Vietnam.
There are currently three direct flights from Seoul to Hanoi organized by Korean Airlines. Two of them are daily with one departing at 19.30 (local time) and another one taking off at (10.05). Another flight on this route is only provided on Monday, Thursday, Friday and Sunday at 18.05 every week. The ticket fare for an adult on Economy class in those flights is approximately $763 for one-way trip and $965 for the round trip, depending on the time you book.
On the other hand, daily flights from Seoul to Ho Chi Minh City have two options. One flight departs at 19.15 and lands at 22.20, and another one takes off at 10.15 and arrives at 13.40. The price for one seat on the Economy class is $763 for one-way trip and $971 for the round trip.