One of the challenges researchers face in trying to learn about the characteristics of vegetarians is that there are so few of them. I've seen estimates that put the percentage of vegetarians in the US population as high as 13%, but most estimates are closer to 5%. That means that if one does a survey that has 1,000 respondents (which is a pretty typical sample size for pollsters), you'll only have about 50 vegetarians in the sample - hardly a large enough sample size to say anything meaningful.
We've been running the Food Demand Survey (FooDS) for 19 months now, and each monthly survey has over 1,000 respondents. I took the first years' data (from July 2013 to July 2015), which consists of responses from over 12,000 individuals. This sample is potentially large enough to begin to make some more comprehensive statements about how vegetarians might differ from meat eaters in the US.
Applying weights to the sample that force the sample to match the population in terms of age, gender, region of residence, etc., we find that 4.2% of respondents say "yes" to the following question: "Are you a vegetarian or a vegan?", which means that 95.8% say "no".
There is some sampling variability from month-to-month, but overall, the trend in the percentage of respondents declaring vegetarian/vegan status has remained relatively constant, and if anything, has trended slightly downward over time.