Jacob Singer may have entered into a Vietnam fantasy within his existing "City World" to relive his wartime memories for some bizarre reason. The reason his platoon becomes impaired is because Jacob is experiencing side effects from his sedative he took. Jacob eventually becomes stabbed in his Vietnam fantasy, and returns to a version of the city world. Jacob is not free of "Vietnam World" because the wound was not immediately fatal to his dream projection.
However, Jacob has become deeply trapped within his dreamscape and starts being visited by an extraction team who is trying to break him out of his coma. "That was another life Jacob, let it lie." They are extractors are probably trying to free Jacob before his brain becomes warped by excessive time in a dream.
The end has Jacob coming to resolution in accepting the death of his son, and leaving the "City World" for somewhere. Maybe Jacob wakes up, maybe Jacob enters another layer to enjoy his resolution for some time?