Hi David,
The OPPO has variable volume feature indeed. This feature is implemented using the volume control setting of the on board DAC, which gets replaced by the VanityHD board. The VanityHD board does not support this functionality. Mainly for the fact that purely digital volume control is perceived as inferior by many people because it reduces the dynamic range of the DA conversion process.
Can you please share the detail which digital amps you use, these things usually have their own volume control. but I understand there can be a synchronization issue. The only thing that can be done in the OPPO is a fixed gain trim in the speaker configuration menu.
Regards,
Pavel