This is certainly an interesting one. See if you agree with Judge Tashima (who writes the majority opinion) or Judge Callahan (who dissents).
I know only one thing for sure. If I had a twelve-year old daughter who called up an adult chat line and had phone sex with (and "sexted") someone she met thereupon, and who then promptly stole money from me to buy a bus ticket to Montana where she had four days of bondage and sadomachochistic sex (plus drug use) with a 6'4" 370-pound adult male who knew that she was twelve and who my daughter in turn knew was a registered sex offender, I would die. Have a heart attack and die. Right then and there. Especially when I learned that this wasn't the first time my "precocious" seventh-grader had used drugs and had sex with older men.
Not exactly a day I'd relish.
As for the law, Judges Tashima and Callahan don't agree on whether the twelve-year old girl in this case was a "vulnerable victim" for purposes of the sentencing guidelines. Both have solid points, and you can probably intuit the competing arguments in this regard.
See which side you think's more solid.
And pray -- as is often the case with Ninth Circuit cases -- that you never have to personally confront the circumstances at issue.