You could also take TK to BKK or SIN via IST. LX via ZRH, SAS via CPH or OS via VIE is another option. BKK-HAN on TG, SIN-HAN on SQ.
LH,TK and UA fly to SGN, not HAN. Flights inside Vietnam are inexpensive when you book directly on the Vietnam Airlines website. You could also fly into SGN and then continue on a paid ticket to HAN (it is about a 1-2 hour flight)