A vector is a position in space relative to other vectors, and by using Pythagoras and some Trigonometry, we can determine the vectors direction and distance in relation to a target vector.
For the projectile, you are going to need to determine the length of its flight path from the start to the end vector and the angle to which you must launch that projectile.
To find the magnitude(direction):
Say we have two vectors in R2(2D Cartesian coordinate system): P(3,4) Q(6,8)
we must subtract each of the corresponding X and Y terms, then subtract those two elements, square them, then take the root of the whole equation, this is illustrated as follows:
which equates to roughly: 2.6458 (Euler angles)
Now we have the distance between the two vectors, it is time to generate the angle:
Which roughly equates to: 1.285
So now rotate your projectile to 1.285 degrees and then offset the projectile for a distance of 2.6458.
Albeit not a complete solution, you can do this check each time the target moves and update the destination of the projectile, you will however, need to find a suitable method to transition between angles, otherwise the result may look choppy.