Theme Options > Footer > Footer Type > select 1 type. Did you create footer as a static block? If yes, you can use VC editor here. To enable VC editor for static blocks go to wp-admin panel > Visual Composer > Role Manager > Post Types > Custom > tick “static block”,
Please read our documentation http://www.8theme.com/demo/docs/legenda/#!/footer and watch – https://www.8theme.com/videos/ (Legenda WordPress Theme – How to create footer).
Feel free to ask if you have any other questions.