It said Vanderhorst was so "intoxicated" on Jan. 19 that he didn't see the pickup truck that was traveling in the opposite direction as he turned into a McDonald's parking lot on West Baltimore Pike.
The impact of the collision forced the truck onto the sidewalk, where it struck DeSimone and Robison, who suffered severe injuries.
Vanderhorst agreed to a blood analysis at the scene. He was arrested Monday after the complete toxicology report became available, the District Attorney's Office said.
"This is an example of how marijuana is a dangerous drug with devastating impacts to the families of those victims," District Attorney Jack Whelan said in a statement.
In addition to the homicide and DUI counts, Vanderhorst was charged with aggravated assault by vehicle, recklessly endangering another person, and driving under the influence of a controlled substance.
He was arraigned in Delaware County Court, where a judge ordered him held without bail.
DeSimone was a popular athlete at Upper Darby High School, where he played ice hockey and lacrosse, overcoming a medical condition that at one point had left him confined to a body cast.
He had hoped to continue his athletic career at Neumman.