You are correct in assuming that the 6528 is only supported by DAQmx, however there is a way to program with this board using Visual Basic 6.0 and DAQmx. (You will need Visual Basic 6.0 sp6 though).
If you use the latest version of the driver NI-DAQ 7.5, you can now create NI-DAQmx applications with Visual Basic 6.0 using a type library created from the NI-DAQmx ANSI C API
Once you have downloaded the driver, it will provide you with a help document explaining how to go about creating VB 6.0 applications with NI-DAQmx
Hope this helps, post back if you have any questions.