What factors or variables increase an adult's vulnerability to drug-facililiated sexual assault?

Using drugs!: In some ways you answered your own question. Vulnerability to sexual assault can arise out of a history of trauma and abuse for the perpetrator as well as the victims. However, this may never happen when they are clear headed and unimpaired. Add drugs or alcohol to the equation and the odds change for both. In fact, use of substances increases the chance for anyone.

Vulnerability: Here's a starter list: Willingness to take any recreational substances, low-self-esteem, confidence, control, discipline. Boredom, loneliness, depression, poor judgement, bad environment,etc. Let's talk of good defenses: Opposites of the above! Find a source of support - from solid, safe activity groups like gym, yoga, meditation -thru- individual or group therapy. Be safe. Be strong. Be sober.