Shortbread is called shortbread because the recipe usually calls for shortening. So technically shortening will work better than butter. It has a higher fat content than butter and it doesn't spread as easily. Shortening shortens dough, which means it makes the texture crumbly.
When it comes to taste, I think vanilla and salt sounds great. I find that butter flavoured margarine usually loses its butter flavour during baking, so I think you'll be fine.
awesome, I will give it a shot. ThankS!