Speakers on the long wall

I had a pair of Vandersteen 2CE a number of years ago. I liked them. 

You should try pulling the speakers out into the room about 3 feet.  Try spacing them about 8 ft apart with your chair about 8 feet away.  If I remember, I had mine towed in a little.  That's just a starting point. 

What type of floor are they on?  Do you have them on stands?  I had Sound Anchors filled with sand. 

I actually prefer having my speakers on a long wall. We were in a rental house for a short time.  The room was only about 10 foot in the short direction. I put the Vandersteens on the short wall and it sounded awful. Like being in a tunnel.  We weren't there long, so I didn't try different placement. 

There is 6 ft between the inside edges of the speakers, and the front of the speaker is 28" away from the wall.

They are on the Sound Lab stands on a cement (carpeted) floor.    

So it literally leaves you 4–5 ft between the MLP and the back wall. If untreated, it can reflect sound waves that smear the image, create standing waves, and reduce depth. In addition to diffusers on the front wall between the speakers, you will need to place absorbers (not diffusers) to treat the back wall. It’s important to note that if there is ample distance behind the MLP, a diffuser is great for retaining spatial cues — but in your case, the space behind you is limited. 

If you want to retain spaciousness without sacrificing clarity, an effective compromise is to place absorbers on the lower half of the back wall and diffusers on the upper half.

I've got a long living room that I've partitioned to have my speakers against the long wall, and I've essentially no side walls to reflect sound as they are so far away. I've got my speakers about 18" from the wall and about 8 feet between each other and between them and my listening chair. Combine that with a high ceiling and wall-to-wall carpet and the room contributes as little as possible to the sound.