Using WP3.9beta3 and Visual Composer (v3.7.4 or 4.0.4):
Editing content in the normal WP editor window seems to work as advertised.
Editing text in a wysiwyg window of VC is horribly broken (and yes, we do have a support ticket to the plugin author). HTML tags are rendered in visual mode (even <p> tags, which don't show in text mode), text is white on a white background, the button bar doesn't show up, and edits are not applied in VC 4.0.4. Maybe the "visual" rendering/styling isn't being applied properly?
We see reports of similar problems affecting the Advanced Custom Fields plugin:
TinyMCE seems to be a common thread in the difficult interaction between WP3.9 and some very important and popular plugins. Hopefully this will be fixed before release, otherwise many sites will break.