Hanoi to SFO through Seoul
 

Hanoi to San Francisco going through Seoul. Do I go through customs in Seoul? only a 2 hour lay over before flight to SFO. Will I need to pickup bags and go through customs and recheck in in Seoul

If the flights are on the same ticket, no, your bags will go through and you won't go through Korean border controls. If they're on separate tickets, yes, you would, and 2 hours probably wouldn't be enough.