#4. I think Mike goes back to Vietnam to find Nicky because before they left for Vietnam together, Nicky asked Mike to promise that he will bring Nicky back home no matter what happens and no matter what condition Nicky is in. Mike tried to get Nicky's attention when he was in Vietnam but Nicky drove away in a car so Mike just came home. After a while, when Mike decided to go deer hunting, he couldn't kill the deer he was aiming at because Nicky wasn't there, and that's when Mike decided to go back to Vietnam to find Nicky and bring him back home, because Mike realized that he cannot do the daily things he used to do without thinking about what he promised Nicky.