Toslink or Coaxial from Bluesound Vault 2 to external DAC?

not a fan of his, BUT was it Redbook source material?

it has never been clear to me if the nebulous negativity towards Toslink is based on the interface per se, on scratched up connectors, or... on some animus towards something widely used on A/V gear - only the former is a valid concern overall; only the 2nd is a valid concern for a particular implementation

It is system dependent - what works for him might not work for you. Toslink doesn't make ground loops and is free from electrical noise pickup or reflections in the cable, but is sensitive to system noise because transitions are slow.  Slow transition in presence of the system noise can cross threshold point at different times creating time jitter that converts to noise and loss of clarity.