I'm pretty sure that the y shaped piece just slips over the end of the shock and gets bolted through where the bushing usually goes. I'm building up my friend's new one this weekend. I'll check it out and get back to you.
Unfortunately, putting a Vivid Air on a 2011 Demo 8 is not a simple task. In stock configuration, the air inlet contacts the frame preventing proper alignment. In order to get the Vivid Air to fit, the shock needs to be modified (unless this mod was done by Rock Shox in the 2012 Vivid Air Models) but neither Specialized nor Rock Shox recommends this and most likely will void any warranty issues resulting from the modification. I know you saw the photos in Pink Bike, but at that time, that Vivid Air was not stock. The modification is not difficult and if you take a look at the Vivid Air Tech Manual, and the exploded view of the shock internals you can figure out what to do. But do you really want to do it to a brand new $600 shock and risk not doing it correctly?
Also, the Shock Extension (or Y shaped piece) that comes stock on the frame is specifically for the Fox shocks. You will need a Vivid shock extension and even with that, the extension will need to be modified to accommodate the Vivid Air rebound knob. My recommendation is not try to modify a Vivid Air to fit the 2011 Demo 8 frame.
I did this. I was told how to modify the shock to make it fit. I did this and it worked great for a few rides. but it didnt last. the shock lost all pressure. Got a new one and tried again. same thing. I Saw the sram guys and they said i was doing it wrong and they dont suggest trying it yourself. that being said. they did take my shock and did what needed to be done and its been awesome ever since. When I asked them what they did they would not tell me because they didnt want it to be done. also the "Y" extender needs to be modified.
I love my vivid air to death but in all honesty, if something happens to this shock I will not go thru the hassle again.