You can’t find Vista Point: Dunehollow in Horizon Forbidden West until you’ve completed the Sea of Sands main quest, thereby draining all of the water out of Dunehollow. But once that’s done, you can re-enter Dunehollow (for the Night of Lights errand quest, perhaps). Use your Focus inside the Dunehollow dome and you’ll see an “unidentified signal”. This marks the base of the Vista Point: Dunehollow tower, so get up close and use your Focus again to receive the image fragment.
Vista Point: Dunehollow itself is at the north end of the Dunehollow dome. Head up that way and you’ll see half a bridge (Aloy calls this an “overhang”) with a ladder hanging off the end of it. Climb the ladder, then turn to the left and step onto the rough-shaped bit of bridge sticking out to one side. Look south and call up the image fragment to find Vista Point: Dunehollow.
Related: How to find Vista Point: The Long Coast in Horizon Forbidden West
The Vista Point: Dunehollow Datapoint boasts of the Vegas Strip’s water filtration and recycling system, but mentions nothing of the other stuff ancient Las Vegas was famous for, like gambling, entertainment, and other stuff that happens in Vegas and definitely stays in Vegas. You’ll have to use your imagination to fill in the blanks.
