Set the vterm buffer's major mode before displaying the buffer.

This ensures that display-buffer actions that condition on a buffer's
major mode (see the documentation of `display-buffer-alist` and
`buffer-match-p`) properly catch the buffer when it is displayed for the
first time.

This aligns with the behavior of other terminal emulator
modes (e.g. term and ansi-term) and comint-derived modes (e.g. ielm,
SQLi, and inferior Python).

Author: tmurph

vterm.el

@@ -1503,10 +1503,10 @@ value of `vterm-buffer-name'."
                    (t
                     (get-buffer-create vterm-buffer-name)))))
     (cl-assert (and buf (buffer-live-p buf)))
-    (funcall pop-to-buf-fun buf)
     (with-current-buffer buf
       (unless (derived-mode-p 'vterm-mode)
         (vterm-mode)))
+    (funcall pop-to-buf-fun buf)
     buf))
 
 ;;; Internal