You should not use the Crystal DirectSound driver, because it doesn't support hardware mixing.
In the general settings dialog of RME Digi96 there is a Disable DS (Disable DirectSound) checkbox in the group field Safe Mode. If this checkbox is enabled, the RME Digi96 provides an emulated DirectSound driver only. The problem-free DirectSound operation of VSampler requires to disable this box. It's strongly recommended to run the RME Digi96 in ASIO mode anyway.
After changing the number of channels or the channel-type (Mono/Stereo) it's possible Cubase VST can't load existing VSampler-Songs anymore (an error message appears). Solution: open VSampler in an empty project and load + re-save the song.