Well I can tell you the reason for the black images, if your camera background color is set to black and your vector lines are set to black with fill off there's your problem

As for trans on a vector render I can't really help there because I have never done it. Though I beleave that Vector images can't care Alphas though I could be wrong there. Off the top of my head though I would try this first to see if it worked....

Uncheck Fill object
Check Include Edges
Turn Edge Style to Entire Mesh
Make sure to leave Hidden Edges un-checked
Set your camera background color to a bright green or something that will stand out from your lines
Render as a Tiff sequence
Use After Effects's Color Key or Create a PhotoShop action for all of the images and batch render it out of PS that will cut out the Green leaving the wireframe with an alpha.

You may have to render at a higher res then your final to make this work then just scale it down. I am sure there is a better way though....

D.