From (elisp) Display Tables:

--8<---------------cut here---------------start------------->8---
   The ordinary elements of the display table are indexed by character
codes; the element at index C says how to display the character code C.
The value should be ‘nil’ (which means to display the character C
according to the usual display conventions; *note Usual Display::), or a
vector of glyph codes (which means to display the character C as those
glyphs; *note Glyphs::).

[...]

   The display table also has six “extra slots” which serve special
purposes.  Here is a table of their meanings; ‘nil’ in any slot means to
use the default for that slot, as stated below.

[...]

5
     The glyph used to draw the border between side-by-side windows (the
     default is ‘|’).  *Note Splitting Windows::.  This currently has
     effect only on text terminals; on graphical terminals, if vertical
     scroll bars are supported and in use, a scroll bar separates the
     two windows, and if there are no vertical scroll bars and no
     dividers (*note Window Dividers::), Emacs uses a thin line to
     indicate the border.
--8<---------------cut here---------------end--------------->8---

But none of the following work for me:

--8<---------------cut here---------------start------------->8---
(set-display-table-slot standard-display-table 5 ?|)
(set-display-table-slot standard-display-table 5 (make-glyph-code ?|))
(set-display-table-slot standard-display-table 'vertical-border ?|)
(set-display-table-slot standard-display-table 'vertical-border
                        (make-glyph-code ?|))
--8<---------------cut here---------------end--------------->8---

And I'm pretty much sure that this happens because the character '|' is
hardcoded in "build_frame_matrix_from_leaf_window" in "src/dispnew.c".

-- 
Akib Azmain Turja

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