How to make org-capture open buffers in new vertical split window in emacs?

How can I modify org-capture's behavior and make it place the newly opened buffer, after selecting a template, in a new vertically split window on Emacs? More precisely, how to make capture's template window be placed below (split-window-below) current focused or leftmost window?

This is a complicated subject (which I don't understand completely - caveat emptor!). The problem is that there is a long conceptual distance between org-capture and the function that actually does the window splitting, a function called split-window-sensibly. So there are many places where you could conceivably interject a change in the behavior, but the trouble is that whatever you do that way might break a lot of other things that have nothing to do with capture.

The doc string for split-window-sensibly (do C-h f split-window-sensibly RET to read it) does mention two variables however:

By default display-buffer routines call this function to split the largest or least recently used window. To change the default customize the option split-window-preferred-function.

You can enforce this function to not split WINDOW horizontally, by setting (or binding) the variable split-width-threshold to nil. If, in addition, you set split-height-threshold to zero, chances increase that this function does split WINDOW vertically.

In order to not split WINDOW vertically, set (or bind) the variable split-height-threshold to nil. Additionally, you can set `split-width-threshold' to zero to make a horizontal split more likely to occur.

So I would recommend that you define your own org-capture function that sets these variables using a let-bind before calling the "real" org-capture:

(defun my-org-capture ()
  (interactive)
  (let ((split-width-threshold nil)
        (split-height-threshold 0))
     (org-capture)))

and use it instead of the "real" one. E.g. you can bind it to what the Org mode manual recommends by doing

     (global-set-key (kbd "C-c c") 'my-org-capture)

(or modify whatever key binding you use).

The advantage of this is that it only modifies how you call org-capture, so there is virtually no chance of breaking anything else. And you can easily undo the change if necessary.