I think you'll want something like:
select n.nspname, c.relname
from pg_class c
inner join pg_namespace n on c.relnamespace = n.oid
where reltoastrelid = (
select oid
from pg_class
where relname = 'pg_toast_15404513'
and relnamespace = (SELECT n2.oid FROM pg_namespace n2 WHERE n2.nspname = 'pg_toast') )
It'd be nice if Pg reported this in the vacuum command summary.