Would it have made sense if Victoria vaguely knew the guys already...?

Couldn't agree more. The thought of any girl being harassed by four random drunk guys in the street and going along with it isn't too believable.

It's not just the loneliness. Victoria suffered from the "Illusion of Grandeur". This was clear in the piano scene when she wanted to become this amazing extraordinary pianist when she's actually very mediocre. She also lacked self-control, dancing until 4am when she's supposed to work at 7. She also tended to escape from reality, as she ran away to Berlin only because she failed to become a famous pianist. So the heist gave her the chance to feel big, that she's not ordinary, that she could make something of herself that she failed to do in Spain. The fact that she and the guys clicked right away is not unusual, and that familiarity became a catalyst for her to make the rash decision to become the getaway driver.

Why do so many people have a hard time getting her character? Her behaviour makes perfect sense. It was very well shown that Victoria is at a point in her life where she feels disconnected, at a point where she doesn't care much about anything. Throughout the movie we are shown that, and it's not even subtle. It starts with the scene where she tries to make contact with the barkeeper. She's clearly looking for connections. She's also reckless, as we see on the roof when she climbs on the reiling. Even the guys think she might be a little crazy in that moment. Then the piano scene. It tells you pretty much exactly where she is in life. Nowhere. Her dream of becoming a star pianist was crushed. She had to leave her home in order to find a job (as many other spanish people who come to Berlin). She's drifting without a plan for her life, far from home, without any friends. She has nothing to lose, and she clearly feels a connection to these guys, especially Sonne. It's easy for me to understand the choices she makes.