UPDATE
I realised I'd made a mistake in not changing the dynamics controller for all the articulations individually in the Dorico Expression Map as I'm used to just setting the control once in VI Pro itself for the instrument. So my previous post can be largely dismissed as nonsense and apologies for any confusion caused! In fact now, I get the same results setting Volume to CC7 as Note Velocity with the obvious exception that within-note dynamics work with the CC and not velocity. I'm using Vel.XF as secondary controller
This, though, begs the question why Andi advises in another Dorico post for notation software to use velocity as the primary dynamics controller in notation software (other obviously for percussive and other instruments with no post strike control). If I could understand this, then I might be another step on the way to really taming my VSL libraries!