So let's call the coordinates of point A (a, b) and the coordinates of point B (x, y), for fun. Then point X should be at (x-a, y-b).

If you plug numbers into this, you'll see that the x-coordinate of point X will be positive northeast and southeast of point A and negative northwest and southwest. The y-coordinate will be positive northeast and northwest and negative southeast and southwest. I only include this information because it took me like forever to figure it out the last time I had to do something like this in VERGE.

So the formula for finding the internal angle A' is just:

tan A' = opposite length / adjacent length

or:

tan A' = (y - (y - b)) / (x - (x - a))

which reduces I think to

tan A' = b / a

So then to find A, you use the inverse tangent. The function you want in VERGE is atan2, described here: http://verge-rpg.com/docs/view.php?libid=1&function=171

Possible bugs you'll encounter with this include: the problem with situations when point B is directly north, east, west, or south of point A, weird issues with converting between on-screen coordinates and map coordinates (some of the graphics functions use one, some use the other), and above all issues with fixed-point notation. I use some of this stuff in the game I'm working on and I still have some bugs related to it with all of those situations, so, um, I can't really explain how to solve these problems yet. But the stuff above is the basic unrefined math you'd need, I think.