abs() gives you the amount of the speed.
sign() gives you the sign.

Maybe, you can explain what you want to achieve in the end.
As long as you speak of speed, I understand that you want to know an amount, because minus or plus is about the 'direction' of a speed.

To get the differences of whole vector:

vec_set(vector_of_copied_speed, my.pan);
wait(1);
vec_diff(vector_of_speed, vector_of_copied_speed, my.pan);