Vince Fontaine is a radio DJ in the vein of Dick Clark from his American Bandstand days. Fontaine hosts a live dance show called National Bandstand. He's also a grown man who hits on teenage girls and attempts to roofie them.
The guy doesn't make any effort to hide his creeper nature. He says this to Marty,
VINCE: Do your folks know I come into your room every night?
And before we can stop shuddering, Marty later tells Rizzo this,
MARTY: I caught Vince Fontaine trying to put aspirin in my Coke at the dance.
If Vince Fontaine were real, we'd have eventually heard of a Cosby-like scandal surrounded this skeevy man. But he's not, and Grease takes place in the 1950's where no one seems to care about this.