If I want to add staging directions above the stave what is the correct Dorico approach? I need them to appear in the ‘full score’ and ‘vocal score’ layouts, but not in the parts layouts.
Your best bet would probably be to add a text frame to the page itself.
The next question is whether you want to add the text frame to the master page or to the actual layout. The latter option would create a page override (not a bad thing, but completely manual). The rule of thumb is that if you will be assigning any of the same content or design/formatting to multiple layouts, use a master page, and then assign that master page to the layouts.
You will need to make a new master page set for the vocal score layout to differentiate it from the parts layouts. Unless you can use the same master page set for both the full score and vocal score, though I doubt you’d prefer that.
I put the vocal lines at the top of the score (rather than above the strings) and attach the text to the top vocal line. It then translates into the piano/vocal score relatively smoothly. Granted, I am not using extended stage directions. I also sometimes make the text background opaque and have it cover up empty vocal staves.
The greater problem is the occasion when I need more room above the top line and below the page header; it bothers me to lower the top staff (and possibly those below it) so they do not line up across the pages–but I do it if I have to. (After all, if conductors can read cutaway scores…)
All excellent suggestions, thank you.