The studies we did were produced in another life (that's a little convenient I know) and I don't have now...we spent $30,000 to determine the best corporate fonts for printed materials (Book Antiqua for body and Arial/Helvetica for headers, so there, I saved you $30,000
) but for vision impaired, the more contrast the better.
I think, since I can't find the past studies I referenced I'm going to have to eat crow for now.
Time for some homework...I have a study here in my filing cabinet (yikes, paper!) done by some CS students at Dalhousie University indicating in many cases black text on white is better on eye strain for good visual, while the more strained the eyes become, it goes to white text in a dark background...the objective being eye strain management, and contrast.