test.js

var helloWorld = function (callback) {
    setTimeout(function() { callback("Hallo Welt") }, 100); 
}

var vows = require('vows'),
    assert = require('assert');
    
vows
.describe('Hello World')
.addBatch({
    'when asking for hello world asyncly' : {
        topic: function()  { 
            helloWorld(this.callback) 
        },
        'we get "hallo welt"': function (x1) { //<= add a 2nd param to make it work
            assert.equal(x1, 'Hallo Welt');        
        }            
    }
})
.export(module);

its assuming the first argument will be err. if there is no error, the argument is still there, its value is just null

thanks.