using get_option to add a different js

You had an extra ( before get_option and the if statement syntax was wrong. Here is the revised code:

add_action('wp_footer','isotope_vpl_set_style');  

function isotope_vpl_set_style() {   
    $style = get_option('damien_style');    
    if($style == "isotope") {
        include("inc/myfile.js");
    }
}