In the first scene Shawn says, addressing Vista
... I checked her pulse but she was gone. And the scary thing is it reminded me of you and just like you, she left someone behind.
In my first viewing I thought she simply had broken up with Shawn, but him trying to commit suicide through the movie makes more sense if his girlfriend is not alive anymore and is so heartbroken to not want to live anymore.
So, is Vista alive during the events of the movie?