If i run
C-x 3, when emacs starts up it will split window and creates two windows of equal size like this
Now i run
C-x 3 again it will split current window into two parts resulting in this
But whenver i create a vertical split, i want to auto adjust width of each window to equal size like this.
window-configuration-change-hook. However you may want to bind your own split commands for this instead as @InHarmsWay suggested. Using the hook would likely have side effects you don't want with commands that split windows for other purposes.