I had a similar problem at one time using VSampler which did not render properly using an ASIO driver but would do so using the MME drivers. The effect with the ASIO was that the rendered file was blank although RB was happy.

I also had various other problems such as VSampler losing the patch selections and closing the VSTi. I found that if I saved a copy of VSampler setup, then the VSTi did not close and that it was possible to render correctly using the ASIO.