When splitting the Vim window vertically, Vim changes the position.

This is a known problem. When you are splitting the Vim window, Vim will try to draw a scrollbar. Since this changes the gui window, Vim tries to resize its main window to keep the same position and this will cause Vim to move its position. This happens on Windows with a multi-window setup or a window that was “snapped” to a certain position. A workaournd to this problem is, to remove gui scrollbars, e.g.

    :set guioptions-=L