How is Heart of Darkness a vivid travelogue?
Conrad's Heart of Darkness could be seen as a vivid travelogue is many ways.
At the heart of the story you have the tales of two different white men journeying into the African wilderness at two different times. Both Kurtz and Marlow venture up the Congo River for similar and yet different purposes and under similar and yet very different circumstances. Each journey is fraught with danger, from the wildlife, the river itself, the natives, and for the competing interests of the other employees of the company. The di of these journeys and the effects they have on these two men are certainly "vivid" to say the least.
Also, we have the symbolic journey into the evil that resides in the heart of humanity as well. The novella is an exploration of all the corruption of which we humans are capable. The parallel adventures of Marlow and Kurtz echo the parallel physical and spiritual journey each man takes.
Finally, the frame story of the four men sitting on board the Nellie in the mouth of the Thames River waiting to be taken out to see could also qualify as a vivid journey if for no other reason than the descriptions and establishment of the light/dark motif that is employed throughout the story. The "brooding gloom" of London behind them and the vanishing light of day all foreshadow the darkness and evil in the story the other men are about to hear.