I don't know how vinyl sales are faring among high-end audio owners, but among the buying public as a whole they continue to rise: up 89% to 1.9 million in 2008, up 33% to 2.5 million in 2009, up 14% to 2.8 million in 2010, up 39% to 3.9 million in 2011, and up 17.7% to 4.6 million in 2012. These figures are just new vinyl, not used.
That being said, sales didn't rise at nearly the same rate in 2012 as they did in 2011. So, there might be something to your theory about vinyl sales peaking. To really assess that, though, I guess you'd have to compare to digital (CD, MP3, etc.) sales and overall sales for the same time period. Did digital sales and overall record sales also follow similar patterns in 2011 and 2012, or just vinyl?