Is the average income of vegetarians in the US higher than the overall
average income?

I couldn't find any good, hard data from free sources. 

Here's a statement without documentation:

"And as a group, vegetarians are more affluent than non-vegetarians."

http://hermes.hhp.ufl.edu/keepingfit/ARTICLE/veglife.htm

This may be of interest: the mean income of subscribers to "Vegetarian
Times" is $76,000.

http://hermes.hhp.ufl.edu/keepingfit/ARTICLE/veglife.htm

Bugbear --

Pink actually cited the source that I'd have recommended -- the
leading vegetarian magazines.  However, a friend used to be editor of
Vegetarian Times and you may want to scrutinize the numbers a little
-- they'll carry a bias towards types of subscribers and their
demographics.  The former editor of Vegetarian Times characterized the
vegetarian demographics as having a large number of young people
(under age 25) and another node in the 50+ demographics, specifically
people who were interested in a vegetarian diet to combat
health-related problems.

Best regards,

Omnivorous-GA