Okay, so there's a plot twist. What about the truck?
Obviously, the truck wasn't imaginary. Where did the truck come from?
shareOne way of understanding the movie makes the killer and his van all part of Marie's fantasy. As you say, that's very hard to believe since the van/truck is where we find out about the twist and where the supposed real action begins from.
The only other - and presumably better - way to make sense of this stupid stupid movie allows that the van/truck was real and driven by Marie all song. That is, *all* of the opening scenes with Alex and Marie driving to Alex's family, meeting the family, going to bed afterwards, etc. are Marie's fabrications. Really Marie arrives uninvited by Alex at the family's house in the country for the first time late at night, slashing the father when she meets him for the first time at the front door.