you should be able to check your bags all the way providing the 2nd ticket to the gate agent. it all depends on who checks you in. i have had agents who would and would not previously. keep in mind that delta arrives in terminal 2 in ICN and vietnam leaves out of terminal 1. if you stay within the secured area (not leaving the terminal), the trams runs very often and would take you no more than 10 minutes to get from T2 to T1. however, if you have to get out to collect your bags, then you will have to get on a shuttle. that i won't be able to help you out since i have not used that service myself. i tend to stick with the same terminal. good luck!