How to calculate the angle of a vector from the vertical?

Question

Im trying to find out the angle (in degrees) between two 2D vectors. I know I need to use trig but I\'m not too good with it. This is what I\'m trying to work out (the Y axi

Answer 1

It should be :

atan( abs(x1 - x)/abs(y1 - y) ) 

abs stands for absolute (to avoid negative values)