Product.js

module.exports = {
    process: function() {
        Vendor.find().then(function(res){
            console.log('success');
        }).catch(function(err){
           console.log('failure');
        });
    }
}

Product.test.js

var Promise = require('bluebird');
var assert = require('assert');
var sinon = require('sinon');
require('sinon-as-promised')(Promise);

var rewire = require('rewire');
var Product = rewire('product.js');

var vendorStub = {};
Product.__set__({ Vendor: vendorStub });

describe(function(){
    it('should stub Vendor even though Vendor isnt defined with var', function(done){
        vendorStub.find = sinon.stub().resolves(true);
        Product.process().then(function(res){
           assert.ok(res);
        }).catch(function(err){
           done(err);
        });
    });
});