I hate it when a simple question has such an obvious answer, which I provided originally, but then I have to go get the data to support the obvious answer when I'm refuted, but it appears you may need the following info to know the truth about whether or not a vacant house sells for less money.
First off, with reference to the previous answer, anything that gives the buyer information that can improve their negotiating position, will in the end lead to them making a more informed decision. A buyer that makes a more informed decision is considerably more likely to negotiate for something of value. In the case of a vacant home, the message to the buyer is that the seller isn't living in it and that they have to live somewhere else, therefore, the seller is perceived as being at a disadvantage. The value the buyer is going to now add to their negotiations, to their benefit (read lower offer), is that they are solving a problem the seller of a vacant house has, that a seller that occupies their house doesn't have. That invariable leads to a lower offer and a lower selling price.
Second, does it make sense that someone is going to stage a home with furniture if it doesn't result in a higher sale price? Otherwise, what's the benefit of staging, just so that the potential buyer's can perceive what the house might look like with furniture in it? Then why spend the money on staging if it can't result in a higher sale price, or a faster sale (which itself is a net higher sales price because you get your money faster). Case in point, according to the Christian Science Monitor, March 2006, staged homes sell for 7.4 percent more and twice as fast. And this isn't simply because the furniture and appearance that someone is living in it is better than someone actually living in it, it's because the home doesn't appear vacant.
Third, The Appraisal Journal, published by the Appraisal Institute, published an article in June of 2004 where they analyzed this exact issue. Their conclusion, looking at sales data in the Lexington, KY area proved that in fact, vacant houses do sell for less money and take longer to sell. Anyone who asserts otherwise, just doens't have the facts.
I've provided a link to the article in the web reference to this answer. It's a long and arduous read, but the general conclusion is as I stated above. Keep your house looking occupied and you will make more money and sell your home faster than if it's vacant, period.
Best of luck!