M.P.
I don't know if the ear pressure causes the vomiting or not. But those "barf" bags in every seat pocket are there for a reason. I suggest that it's expected that some people will vomit.
Here is a web site that discusses "airplane ear." Vomiting as well as other symptoms such as vertigo are caused the difference in air pressure in the middle ear and the cabin.
http://www.cnn.com/HEALTH/library/airplane-ear/DS00472.html