From Medical College of Wisconsin HealthLink:
But it sounds like you already knew that much.
This STATEMENT ON MOTION SICKNESS includes a seemingly detailed report on the phenomenon (including the interesting news that amphetamine is a useful treatment), but doesn’t address what I believe to be your question, which I perceive as "How is it that conflicting signals from the inner ear and the eye cause the symptoms they do?
manny’s post refers to SDSTAFF Hawk’s column that really does not take it any further, only confirming the relationship between nausea and vertigo.
Further web searches turned up manny of the same as above, but I’ve failed to find the answer (and now that you’ve asked it, I’m curious too).
So, why does vertigo make your tummy do back-flips? Why not an earache instead?
Perhaps Ned is on to something with the body reacting to confusing news by purging the system of possibly newly taken-in contaminants.