If I understood your suggestion correctly, I applied the filter "swap fields" in VirtualDub, but the resulting picture was very blocky...
Finally, I re did every step of the inverse telecine process and converted a 10 second preview of the AVI. The speed seemed fine in both WMP and Vegas. I'll try to convert the whole movie again and upload it in Vegas, maybe it will (inexplicably) work this time.
Also, is it normal that I notice a slight drop in sharpness after the inverse telecine process?