As I understand it, getting a direction vector pointing from point A to point B is B-A:
then you might need to normalize it, if it needs to be unit length.
Tho this could give sharp corners, so maybe you can approximate for he series A, B & C by to take from A to B and B to C, averaging those and optionally normalize that result.
Is that what you where looking for?
Maybe debug-rendering those can show if it is close to what you want?