I suspect we are referring to different things, here. Sorry if I’ll describe something you already know, but I want to make my question clear.
There are various ‘loudness’ control with MIDI instruments. One is Channel Volume (CC7), usually used to adjust the channel fader in the mixer.
Another one is Expression (CC11), often used as a complement to dynamic control (for added volume, changed loudness while maintaining the same timbre/intensity, or to fade to al niente).
Then there is the Dynamic control (usually CC1 or Velocity), changing volume, crossfading between dynamic layers, acting on related parameters like brightness.
CC11 is usually left as a channel control, acting on the MIDI volume, with no interference on the instrument’s internal parameters. CC1 and/or Velocity are directly linked to the instrument’s sound generation. This is what expression maps are codifying.
In most instruments you can decide to control the internal Dynamics parameter either with an instant control (Velocity) or a continuous one (modulation, usually CC1). Dorico allows codifying both. Some instrument can use both (Velocity for initial dynamics, CC1 for its variation over time).
VSL instruments can use the one or the other, not both at the same time. What it demands is written into the presets and patches. So, when Dorico sends the one or the other, a VSL instrument responds to the message it can recognize for its current settings.
My doubt is if having both Velocity and CC1 written in an expression map makes Dorico send both messages at the same time, therefore letting a VSL instrument respond to the one or the other, depending on its settings. Or, is the Primary Dynamics control to prevail, with the Secondary ignored if the instrument is not set to respond to two messages?