Finding Angle Between Vectors

So... I'm working on a rope swing prototype and in short, I need to find an angle between some vectors or if I can can draw a straight-line between three different vectors. I've been searching for ages but can't seem to find any clear solution.

Say I have three vectors, Vector1,Vector2 and Vector3, I need to find the angle between Vector1 and Vector3 and Vector1 and Vector2. If they're the same then I unwrap the rope.

The other way to do this, is just if I can draw one straight line through all three Vectors then I unwrap. Hope I was clear enough. C# is preferable.