If you visit Google trends, you can add multiple search terms and get a display showing how much interest each of these terms has seen on Google - either for the whole world or for some regions, and you can specify either all years or a limited period.
As you can see from the table above (click on the link), until 2011, there was always more interest in "vegetarian" than in "vegan" - until 2011, when the curves starting to overlap each other. And for 2012, the results in the main section ("Search Volume Index") look very good. I'm not expert on using Google Trends, and haven't checked what the difference between Search Volume Index and News Reference Volume means, but the development generally looks good.
Here's an article from last year which covers the same topic:
Vegan diets becoming more popular, more mainstream