I see that your question hasn't been answered in a few days and, most likely, that is because you are asking for "page numbers." The reason why eNotes educators are hesitating is because editions of the book vastly differ; therefore, there is no reason to give page number examples in that it most likely won't match to the edition you are using. In regards to me, I have a large-print edition of this book, so I know our page numbers won't match up. I can, however, give you some of those "particularly vivid, effective descriptions" you ask for and give you a short analysis for each.
First, let's look at the setting of the desert. Oh my, the desert in the novel is so very harsh! Of course, as a result of the harshness of the setting, the name is reminiscent of satan. Further, it is an incredibly vivid description in regards to both migrants and sense of place:
Nothing soft here. This world of spikes and crags was as alien to them [the Mexican crossers] as if they'd suddenly awakened on Mars. They had seen cowboys cut open cacti to find water in the movies, but they didn't know what cactus among the many before them might hold some hope. Men tore their faces open chewing saguaros and prickly pears, leaving gutted plants that looked like animals had torn them apart with their claws. The green here was gray.
Next, we will take a look at some description regarding characters' opinions. There is an interesting description of Border Patrol. They are often portrayed as caring people "with a job to do." The description here shows that it is the government that is to blame:
But the two things that most unify the two sides are each one's deep distrust of its own government, and each side's simmering hatred for the human smugglers, the gangsters who call themselves Coyotes.
Now let's look at a vivid description of setting in a different way: proximity of Mexico to the United States. This proximity allows the poorest of the poor to observe wealth with their own eyes and extend their dreams. Here is a particularly vivid example:
In Iowa City, Omaha, Nutley, Waycross, Metairie, those who survive the northern passage can earn in an hour what it took a long day's work in radioactive chemical Mexican sludge to earn before. ... Mexicans still behind the barbed wire continue to listen to fabulous tales of Los Estados Unidos. They watch drunk and disorderly teens vomit in the streets of Spring-Break-Atlan. They wait tables and mop floors while sailors scream and naked girls dangle from balconies.
Thus, the setting of the desert is more than just a setting, its bleakness is an actual theme of the book (as indicated in the title "a true story"). With its intense heat shown in the context of Urrea's vivid descriptions gives us an amazing picture of bodily pain and suffering: swollen lips, black excrement, cooked organs, sizzling edges of, etc. All living creatures (even the worst of the worst like the snakes and the scorpions) shrink away from the sun's blazing heat and torture.