Full width header image not working

That might look something like this:

/* Vantage Full Width: 100% Width Logo Space */
body.responsive.layout-full #page-wrapper #masthead .full-container {
max-width: 100%;
}
body.responsive.layout-full #page-wrapper #masthead .site-navigation .full-container {
max-width: 1080px;
}
header#masthead .hgroup .logo {
float: none;
}
header#masthead {
padding: 0;
}
.main-navigation {
padding: 0 35px;
}

Thanks for adding the CSS. Chrome Developer confirms your logo is now rendering at 1280, the space available is just a bit bigger than that on most screens. To center your logo please make this change, where I’ve said:

header#masthead .hgroup .logo {
float: none;
}

Change that to:

header#masthead .hgroup .logo {
float: none;
text-align: center;
}

Unfortunately this isn’t possible with Custom CSS, the logo is being outputted in it’s constrained size. If you’re running a child theme you can copy the entire logo function into a functions.php file in the child theme and then safely edit it there:

if(!function_exists('vantage_display_logo')):
/**
 * Display the logo 
 */
function vantage_display_logo(){
	$logo = siteorigin_setting( 'logo_image' );
	$logo = apply_filters('vantage_logo_image_id', $logo);
	if( empty($logo) ) {
		if ( function_exists( 'jetpack_the_site_logo' ) && jetpack_has_site_logo() ) {
			// We'll let Jetpack handle things
			jetpack_the_site_logo();
			return;
		}
		// Just display the site title
		$logo_html = '<h1 class="site-title">'.get_bloginfo( 'name' ).'</h1>';
		$logo_html = apply_filters('vantage_logo_text', $logo_html);
	}
	else {
		// load the logo image
		if(is_array($logo)) {
			list ($src, $height, $width) = $logo;
		}
		else {
			$image = wp_get_attachment_image_src($logo, 'full');
			$src = $image[0];
			$height = $image[2];
			$width = $image[1];
		}
		// Add all the logo attributes
		$logo_attributes = apply_filters('vantage_logo_image_attributes', array(
			'src' => $src,
			'width' => round($width),
			'height' => round($height),
			'alt' => sprintf( __('%s Logo', 'vantage'), get_bloginfo('name') ),
		) );
		if($logo_attributes['width'] > vantage_get_site_width()) {
			// Don't let the width be more than the site width.
			$width = vantage_get_site_width();
			$logo_attributes['height'] = round($logo_attributes['height'] / ($logo_attributes['width'] / $width));
			$logo_attributes['width'] = $width;
		}
		$logo_attributes_str = array();
		if( !empty( $logo_attributes ) ) {
			foreach($logo_attributes as $name => $val) {
				if( empty($val) ) continue;
				$logo_attributes_str[] = $name.'="'.esc_attr($val).'" ';
			}
		}
		$logo_html = apply_filters('vantage_logo_image', '<img '.implode( ' ', $logo_attributes_str ).' />');
	}
	// Echo the image
	echo apply_filters('vantage_logo_html', $logo_html);
}
endif;

Unless you see something go wrong with your logo, which is unlikely, you can safely run this function in your child theme without comparing it to the parent after each upgrade. The if statement that starts this function off:

if(!function_exists('vantage_display_logo')):

is specifically there so we can do just this and re-use the function in a child theme.

The function is written like this to help optimise images. Keep in mind you’re now customising the theme. 99.9% of use cases won’t involve a logo bigger than 1080px so there isn’t any point in loading one bigger, that’s the reasoning there.

Hope this helps.