Generally speaking, NYPD sitting in a van are on some type of OT detail (extra foot patrols for the type of stuff that MG said, parades, street fairs, etc.). Talking to a pretty girl on the street then walking somewhere together is not going to get them out of that nice warm van. Raids & stings are usually staged away from the target by plain-clothes cops - you're not going to see them coming.
Still, a van full of cops in plain view is always a good crime deterrent. Thrill-seekers might pick up a working girl in view of them. I wouldn't, but that's me.