Fuel uses the file fuel/application/views/_admin/_fuel_preview.php to preview. you can configure it to use the layout you want it to use, by default it is configured to use $this->load->view('_layouts/main', $vars);
There is also a "preview" parameter you can set in your form field to specify a different view file. It must be located in the fuel/application/views/_admin folder. http://docs.getfuelcms.com/general/forms#wysiwyg
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fuel/application/views/_admin/_fuel_preview.php
to preview. you can configure it to use the layout you want it to use, by default it is configured to use $this->load->view('_layouts/main', $vars);
Hope this helps.
http://docs.getfuelcms.com/general/forms#wysiwyg