"I don't buy Vandersloots innocence. He's done nothing but lie, starting with a lie that sent two innocent men to jail. If he didn't do it he knows something."
For sure. I believe it's more than "he knows something". For him to lie over and over, he would have to be involved with her death or disposal. (IMO)
Most likeley two crimes were committed.
Vandersloot has sex with her and abandoned her on the beach. He was afraid of being accused of rape.
The new suspect most likely raped and killed her. And Daddy cleaned up all the loose ends.