How to call ajax in plugin file

Use the built-in wordpress Ajax actions:

Your jquery will look like this:

$('#ajax_form').bind('submit', function() {
    var form = $('#ajax_form');
    var data = form.serialize();
    data.action = 'MyPlugin_GetVars'
    $.post('/wp-admin/admin-ajax.php', data, function(response) {
        alert(response);           
    });
return false; 

Your plugin code something like:

add_action("wp_ajax_MyPlugin_GetVars", "MyPlugin_GetVars");
add_action("wp_ajax_nopriv_MyPlugin_GetVars", "MyPlugin_GetVars");

function MyPlugin_GetVars(){
    global $wpdb;
    // use $wpdb to do your inserting

    //Do your ajax stuff here
    // You could do include('/wp-content/plugins/test/getvars.php') but you should
    // just avoid that and move the code into this function
}

For more info please visit AJAX in Plugins