\new Staff \with {
\RemoveAllEmptyStaves
}
%%%%%%%%%%%%%%%%%
did the trick (along with the afore-mentioned "\bar" and "\break' at the end of the "short" measure at the end of the piano "solo" introduction and before the partial measure where the voice starts.
Frenched usually refers to omitting whole empty systems and is used to
make symphonic scores more compact. Just omitting a staff for a few bars
is not generally thought of as Frenching, technically speaking.
Someone else brought up "French". I'd never heard of it til it was mentioned in a response but it's good to know the terms, even if I'm not doing symphonic stuff, just simple folk melodies and occasionally acoustic rock.