I agree with fhw's advice. I wouldn't worry too much about the dimensions of the shelf space as long as it provides enough room to easily move the speaker in and out.
On the other hand, even though the VP150 is a sealed design, it will still interact with the surfaces around it, so I wouldn't build the shelf area exactly to size. I'd leave at least a few inches of space behind the VP150 for "tweaking room." If you discover that you don't like how the speaker sounds with the extra space behind it, you can fill the space with foam or other sound absorbing material.
