How to get the vector between two vectors?

Again, I’ll remind that I might be missing something, but I think this is pretty simple addition of vectors:

let point A be (700, 500)
let point B be (400, 400)
let point C be (650, 100)
let point D be (???, ???)

the vector from A to B is: (-300, -100) // i.e. x = B-A, 400 - 700, etc
the vector from A to C is: (-50, -400)
Adding these together yields the vector from A to D: (-350, -500).
Adding that vector to point A yields the coordinates of the point D: (350, 0)