The 0.6v setting is primarily in place for use with Airplay. I don’t know of anyone using it for normal listening.
You have too much gain in your system for what you’re trying to accomplish using any normal settings so you need a volume control somewhere. You can either use the VC in the REF6 as intended (being close to unity gain isn’t really critical since the VC isn’t adjusting gain, it’s just attenuating the signal) or you can set the REF6 to unity (or theater bypass) and use the VC on the Vivaldi.
I run without a preamp so I obviously use the VC in the Vivaldi. I have zero complaints with this setup and have no desire to add a preamp.