The House that Vanished
(1974)

 A young model, Valerie (Andrea Allan), is taken by her boyfriend, Terry (Alex Leppard), at night to a deserted mansion out in the woods.  

Terry tells her to wait in the car.  She suspects he is there to steal something.  She gets bored of waiting and goes in.  Unexpectedly, a couple enters the house.  The man stabs the woman to death.  Valerie is horrified, and runs out.  She hitches a ride home.

The next morning, Terry's car is parked in front of her apartment, but Terry is missing.  Her friends tell her not to go to the police, as Terry has a history of disappearing, and of involvement in illegal activity.  A strange man moves into the apartment below her.  He watches and follows Valerie.  Valerie is convinced he is the killer, stalking her.  Valerie tries to take her friend back to the house to look for Terry, but they can never find it.

Valerie's friend Paul (Karl Lanchbury) invites her to spend a weekend away. Valerie is shocked that he has taken her to the house of the murder.  She finds Terry's dead body in a closet.  Paul's aunt shows up at the house.  The aunt knows Paul has been bringing prostitutes to the house and killing them.  The Aunt is angry that he has brought Valerie, and she tells Paul he must kill Valerie.  Instead Paul stabs the Aunt to death.  The creepy neighbor from the apartment below shows up.  He is an undercover cop who has been trailing the killer.  He rescues Valerie.

Thanks Joseph C!