http://www.flowresearch.com/articles...0I%202_014.PDF
This might be a good read.
From the article:
"Low flowrates present a problem for vortex flow meters because they generate vortices irregularly under low flow conditions."
Highly accurate flowmeters are notoriously expensive, and low-accuracy ones won't be much help for flow rates this low.
I'm sure there's a good one out there somewhere, though.
Alchemy