Does the vulnerable victim enhancement apply when the defendant did not exploit the victim's unusual vulnerability?

Questions Presented Section 3A1.1(b) of the Sentencing Guidelines provides for a two-level increase where, “the defendant knew or should have known that a victim of the offense was a vulnerable victim.” The application note defines a vulnerable victim as an individual “who is unusually vulnerable due to age, physical or mental condition, or who is otherwise particularly susceptible to the criminal conduct.” I. Does the defendant have to exploit the victim’s unusual vulnerability in some way for the vulnerable victim enhancement to apply as held by the Second, Third, and Eleventh Circuits or is it sufficient that a victim just happens to have an unusual vulnerability as held by the First and Eighth Circuits? II. Must the vulnerability be unusual compared to the general population, as suggested by the Eleventh and Second Circuits, to other individuals with the same characteristics, as held by the First, Eighth, and Tenth Circuits, or to typical victims of the crime as held by the Ninth Circuit? II. Is the vulnerable victim enhancement unconstitutionally vague under the Eighth Amendment? i