Formation of a small volume of concentrated urine in dehydration is associated with:
In the presence of enough vasopressin to produce maximal antidiuresis, water moves out of the hypotonic fluid entering the cortical collecting ducts into the interstitium of the cortex, and the tubular fluid becomes isotonic. In this manner, as much as 10% of the filtered water is removed. The isotonic fluid then enters the medullary collecting ducts with a TF/P inulin of about 20.
An additional 4.7% or more of the filtrate is reabsorbed into the hypertonic interstitium of the medulla, producing a concentrated urine with a TF/P inulin of over 300. 
In humans, the osmolality of urine may reach 1400 mOsm/kg of H2O, almost five times the osmolality of plasma, with a total of 99.7% of the filtered water being reabsorbed.
The basic requirements for forming a concentrated urine are:

A high level of ADH, which increases the perme­ability of the distal tubules and collecting ducts to water, thereby allowing these tubular segments to avidly reab­sorb water. 
A high osmolarity of the renal medullary interstitial fluid, which provides the osmotic gradient necessary for water reabsorption to occur in the presence of high levels of ADH.

Reference: Ganong’s Review of Medical Physiology T W E N T Y - F I F T H   E D I T I O N page no 685, Guyton  physiology 13th ed page no 374
More reabsorption in the PCT
More concentration in loop of Henle
More reabsorption of DCT and collecting ducts
All of these