Make inclusion of vi mode in prompt togglable.

docs/source/whatsnew/pr/prompt_includes_vi_mode.rst

@@ -0,0 +1,5 b''
+In vi editing mode, whether the prompt includes the current vi mode can now be configured
+-----------------------------------------------------------------------------------------
+
+Set the ``TerminalInteractiveShell.prompt_includes_vi_mode`` to a boolean value
+(default: True) to control this feature.

IPython/terminal/interactiveshell.py

@@ -225,6 +225,10 b' class TerminalInteractiveShell(InteractiveShell):'
         help="Allows to enable/disable the prompt toolkit history search"
     ).tag(config=True)
 
+    prompt_includes_vi_mode = Bool(True,
+        help="Display the current vi mode (when using vi editing mode)."
+    ).tag(config=True)
+
     @observe('term_title')
     def init_term_title(self, change=None):
         # Enable or disable the terminal title.

IPython/terminal/prompts.py

@@ -14,9 +14,8 b' class Prompts(object):'
         self.shell = shell
 
     def vi_mode(self):
-        if not hasattr(self.shell.pt_app, 'editing_mode'):
-            return ''
-        if self.shell.pt_app.editing_mode == 'VI':
+        if (getattr(self.shell.pt_app, 'editing_mode', None) == 'VI'
+                and self.shell.prompt_includes_vi_mode):
             return '['+str(self.shell.pt_app.app.vi_state.input_mode)[3:6]+'] '
         return ''