Thanks Rich.
I guess I got unlucky with the "simple" package I picked to test with.
After adding those dependencies, it adds Vim to the Apps menu, but selecting it only momentarily flashes up a terminal, starting terminal then entering "vim" has the same error as before.
It also adds GVim to the menu - despite not having added gvim.tcz itself (looks like there's desktop files in gvim-base that should probably be in the vim and gvim extensions), so I'm guessing there may be further dependency mixups causing the failure.
Is there a way to see actual errors, rather than just "not found"?
(Fulfilling gvim dependencies involves gtk3 so I'd rather avoid that rabbit hole.)
I tested with various other extensions and dependencies and they've all worked fine.