Hi friend! .
Here are some my advices .
I think You should go to Hồ Chí Minh city or Hà Nội . But I think the best choice is Hà Nội because it is the centre of Vietnam it is so big and there 're a lot of traditional food there . You can also try Bún Cha & drink beer at some coner at night & ẹnjoy the fresh air there .You should bring some short clothes because it's so hot in the summer. You can visit Gươm lake & Old quater ...they're so beautiful .
Bye ,see you soon in Hn .