Sprite.VertexX(1, 1) returns the displacement of the point from it's normal grid position, so if the point is unmoved it will be zero. If the point is moved then it returns the displacement relative to the size of the sprite. So if the sprite is 128x128 and you set the relative displacement of (1,1) to -128,0 then then Sprite.VertexX(1, 1) would return -1.
To calculate the absolute position of any vertex at (column, row) use the following:
the above expressions do not work if the sprite is rotated.